[fdio] Remove redundant ordinal definitions

Test: No behavior change
Change-Id: Id0d174229deda2eecdb3ba5ccdfc028053f20d37

[sync] Apply C API rubric to libsync

Test: No behavior change
Change-Id: I30a413f7fb5d3f4e741e5204532b82e73595c086

[fidl] Use autogenerated fuchsia-io header

Remove hand-rolled verison.

Test: CQ

ZX-1919 #comment In Progress

Change-Id: I8816a9616067e4d437385db4d552d385e33eb975

[async] Publish constant for async loop configuration.

First step towards disallowing NULL being passed to
async_loop_create() and async::Loop()

This adds a new constant and provides an alternative to the
existing one to be consistent with naming.

It also updates all client calls within zircon.

Test:CQ
Change-Id: I93c53e9b575248293dd5bfeea6269281fc329aa6

[async] Change async_t -> async_dispatcher_t

Change name of dispatcher type from async_t to async_dispatcher_t
Keep typedef of async_t for compatibility

Change async_{get,set}_default() to async_{get,set}_default_dispatcher()
Again, keep compatible apis until other layers have been modified to use
new api.

Bug:ZX-2337
Test:CQ
Change-Id: If0b5d061fd114d60e23078dbb5f2a6ed9ed6f6cc

[fdio] Move headers into lib/fdio/...

Change-Id: Ie8d74e716da913bf6e2672c4acf8cd67b4962b7f

[fdio] Regenerate I/O bindings to use fuchsia_io prefix

Change-Id: I444cda2448ca2a560abb7a38513f40005ac2fdce

[fs] Add a managed subclass of Vfs capable of terminating synchronously

Change-Id: I56bf34ed1c25dee556c8d0ad0e12282a9e4752ef

[fidl][io] Regenerate, update usage of io bindings

Change-Id: I9990f4c98a2c57ee8d3e953093b7be7e2d4c71e4

[minfs][blobfs] Re-implement filesystem teardown

Rather than simply "sync + exit process", implement
a fully featured filesystem teardown procedure, capable
of:
- Tearing down all active connections.
- Syncing underlying media using a non-blocking dispatch loop.
- Use closures to signal both mounters and unmounters on
filesystem destruction.

As a consequence of the controlled filesystem teardown
procedure, make the "filesystem" objects in minfs and blobfs
no longer refcounted, to clarify that "Shutdown" is the
single point of destruction after mounting.

ZX-1577 #done
ZX-1923 #done
ZX-1959 #done
ZX-1576 #comment In Progress

Change-Id: I3f8d71af76e391c41c665434baf86a245a7cd223

[fs] Re-implement ManagedVfs for async callback safety.

Multiple race conditions could occur while using ManagedVfs:
- Tearing down connections racing with asynchronous callbacks.
- Destroying the ManagedVfs racing with tearing down connections (which
may be serviced by async callbacks, without accounting).
- Destroying the ManagedVfs racing with torn down/newly registered
connections.

To resolve these issues, provide an explicit "ShutDown" API which will
invoke a closure on behalf of the caller when all Connections (and their
corresponding Vnodes) have terminated. The ShutDown implementation
signals all local channels to terminate, which eventually results in all
Connections tearing down.

Simulate these race conditions and test their solutions using a custom
Vnode.

ZX-1576 #comment In Progress

Change-Id: Id3850e3fec55e1417e79ae9417ec1c5d1c818fe8