IMAP: sync fixes

This patch should fix:

- deadlocks while manual syncing/shutdown;
- crashes while manual syncing/shutdown.

Change-Id: I0962ff5580c19cbf740d002b6f62681ed5f558ab
Reviewed-on: https://review.haiku-os.org/758
Reviewed-by: Stephan Aßmus <superstippi@gmx.de>

mail_daemon: IMAP delete messages on the server only when delete_remote_when_local setting is set

Change-Id: I88fcc7a185cede080af7c3b85f9466a02f895809
Reviewed-on: https://review.haiku-os.org/674
Reviewed-by: Stephan Aßmus <superstippi@gmx.de>

BMailProtocol: Removed delete/append methods.

* The mail system is file based which makes these methods pretty much
superfluous.

IMAP: setting flags, and body fetching works now.

* The direct methods in BMailProtocol now forward the request to the
looper; it's no longer the mail_daemon's responsibility to know
anything about that protocol.
* It's in desperate need of refactoring, but it doesn't hurt to add
it to the repository as is.

IMAP: downloading mails is now working.

* Changed the way the attributes are written to make sure that everything
that can be written once is in fact written just once.
* The rename code in BMailProtocol::_ProcessFetchedHeader() was broken,
and caused the hang of the last commit.

IMAP: work in progress of downloading the mail body.

* Most things are in place now, we just try to download the body to the
wrong file, as the final location is currently unknown.
* Added local only kPartialMessage flag for mails, but it's not being
used yet.

IMAP: sync when requested.

imap: Fetching headers is now working.

* No state is stored yet, though, so that all messages will be downloaded
over and over again.

imap: Testing for new messages is now working.

* There is now a CheckSubscribedFoldersCommand that is issued on the main
connection that triggers everything.
* The new CheckMailboxesCommand divides the new mail check into several
portions, and requeues itself until the next step, ie. the
FetchHeadersCommand.
* The headers of the new mails are downloaded, but nothing is done with
them yet.
* The actual check for the new mails doesn't scale that well yet, not sure
how to properly do this without having to rely on the mail indices. Might
be sensible to solve this via some simple heuristic.

imap: Fleshed out command processing.

* Still doesn't do anything useful, though.

imap: Work in progress of connection setup.

* SelectCommand now correctly encodes the mailbox name again.
* Hierarchy separators from the LIST/LSUB commands are now properly parsed,
and taken into account.
* Folders in the file system are now created for mailboxes on the server,
supporting hierarchical mailbox names.
* A connection worker now has an IMAPFolder that handles the local side of the
mailbox, and an IMAPMailbox that handles the server side.
* Connection workers are now created/deleted, and setup correctly. They will
now also wait in case they don't have a mailbox until they get some.

IMAP: The connection workers are now started.

* On SyncMessages(), the main connection worker is started if it is not already
running (it may only run already in idle mode).
* This will then list the subscribed folders, and create as many more connection
workers as allowed and make sense.
* Finally, it will distribute the folders to the workers which don't do anything
further yet.

IMAP: Early work in progress of main protocol class.

IMAP: downloading mails is now working.

* Changed the way the attributes are written to make sure that everything
that can be written once is in fact written just once.
* The rename code in BMailProtocol::_ProcessFetchedHeader() was broken,
and caused the hang of the last commit.

IMAP: work in progress of downloading the mail body.

* Most things are in place now, we just try to download the body to the
wrong file, as the final location is currently unknown.
* Added local only kPartialMessage flag for mails, but it's not being
used yet.

IMAP: sync when requested.

imap: Fetching headers is now working.

* No state is stored yet, though, so that all messages will be downloaded
over and over again.

imap: Testing for new messages is now working.

* There is now a CheckSubscribedFoldersCommand that is issued on the main
connection that triggers everything.
* The new CheckMailboxesCommand divides the new mail check into several
portions, and requeues itself until the next step, ie. the
FetchHeadersCommand.
* The headers of the new mails are downloaded, but nothing is done with
them yet.
* The actual check for the new mails doesn't scale that well yet, not sure
how to properly do this without having to rely on the mail indices. Might
be sensible to solve this via some simple heuristic.

imap: Fleshed out command processing.

* Still doesn't do anything useful, though.

imap: Work in progress of connection setup.

* SelectCommand now correctly encodes the mailbox name again.
* Hierarchy separators from the LIST/LSUB commands are now properly parsed,
and taken into account.
* Folders in the file system are now created for mailboxes on the server,
supporting hierarchical mailbox names.
* A connection worker now has an IMAPFolder that handles the local side of the
mailbox, and an IMAPMailbox that handles the server side.
* Connection workers are now created/deleted, and setup correctly. They will
now also wait in case they don't have a mailbox until they get some.

IMAP: The connection workers are now started.

* On SyncMessages(), the main connection worker is started if it is not already
running (it may only run already in idle mode).
* This will then list the subscribed folders, and create as many more connection
workers as allowed and make sense.
* Finally, it will distribute the folders to the workers which don't do anything
further yet.

IMAP: Early work in progress of main protocol class.