NetServices: format code using `haiku-format`

This commit formats all the netservices2 code with the `haiku-format` tool from
https://github.com/owenca/haiku-format (commit aa7408e), with the following
customizations:
* SpaceBeforeRangeBasedForLoopColon is set to false
* Braces before a catch block are not wrapped
* Most headers, except for ExclusiveBorrow.h, have been manually reformatted
to adhere to Haiku's header format (issue #19 in the repository)

Change-Id: I693c4515cf26402e48f35d1213ab6d5fcf14bd1e

NetServices: use BBorrow<BDataIO> for custom body targets

Change-Id: Ib2d4b0ca3689338d906f943295278c086c6f2c83

NetServices: Implement controls on the number of concurrent requests.

This change allows the user to control how many concurrent request can be done
per session. This is going to be helpful to running the tests as well; they were
all fired up in parallel, which sometimes leads to our test server refusing a
connection.

Change-Id: I4f1f40b28b8e0199ea5589b36cd8d00ecd849a63

NetServices: Implement asynchronous status update messages.

The integration PostTest has a basic test that the expected messages are sent and
have the expected data fields. The gist is documented in book.dox.

To do are the messages around SSL. However, that functionality is also not
implemented yet, so there is nothing to send.

Change-Id: Ib8f36ed32f9854d643d8256338b71af7067059f0

NetServices: Implement BHttpSession::Cancel()

Change-Id: Iff0a7726e57f3e6bd4e9d0ebac08a370d25a62d7

NetServices: Implement BHttpStatusCode, BHttpStatusClass and Redirects

The user of the API can set whether redirects should be followed, and if so,
how many. This is part of the BHttpRequest API. The BHttpSession then follows
those instructions, and executes the maximum number of redirects the user
would like to follow.

As part of this commit, the BHttpStatusClass and BHttpStatusCodes helper enums
have been added, to give a friendlier access to HTTP status codes and status
classes.

Change-Id: Ic8c9e3fda158e2cce549c8f1d360951f7ac83311

NetServices: Introduce BHttpRequestStream and abstract interface.

This supports asynchronous transfers of Http Requests to a network
interface.

Change-Id: I845fb2e08160d219f85b7a08d2d8872ac7359b47

NetServices: implement hostname resolution and connection for BHttpRequest

BHttpSession::Execute() moves the request into the session, and returns a future BHttpResponse
object. Currently implemented are resolving the hostname, and opening the connection.

There is some scaffolding for the actual data transfer.

Change-Id: I5a8a7a7f8680036b91cdba4beee140bbed6bfd5a

netservices: Flesh out BHttpSession::Impl data and control structures

Change-Id: I4230570ea8339b12d855f7de7fa72e4664013e2d

NetServices: Add skeleton BHttpSession class and its documentation

Change-Id: Ia8a35588422908f6fe9b839ce239fb478baf298b