Everything is cancellable. In fiber_cancel() use fiber_wakeup()
instead of a direct call. This make cancel safe with
regards to wakeup() and timeouts: i.e. it's safe to cancel
a fiber which is waiting on a timeout or is about to get
scheduled.

Avoid an unnecessary interlocked instruction or pipe write
in fiber_wakeup(), it's always called from the same cord
as the one to which the event loop belongs.

Use ev_feed_event() with EV_CUSTOM instead.

Rename Factory to Engine, Factory was a name we couldn't swallow.

Introduce Handler class to disambiguate Engine from Space
(hello, MySQL).

Remove the branch which saves a snapshot without a fork.

Build Sophia before trying to link with it.

Conflicts:
	src/box/memtx_hash.cc
	test/unit/CMakeLists.txt

Conflicts:
	test/box/box.net.box.result

set_snapshot_lsn is called at server start,
and snapshot() API is used to save a snapshot.

Conflicts:
	src/box/box.cc
	src/box/engine.cc
	src/box/recovery.cc
	src/box/recovery.h

Now wal_watcher can be used in a threaded context, make it a
member of struct recovery.
Begin changing recover_remaining_wals to use exceptions.