darwin: create fsevents thread on demand

* Move CF run loop code to fsevents.c.

* Create the fsevents thread on demand rather than at startup.

* Remove use of ACCESS_ONCE. All accesses to loop->cf_loop are
  protected by full memory barriers so no reordering can take place.

Fixes #872.

Conflicts:
	src/unix/darwin.c
fsevents: FSEvents is most likely not thread-safe

Perform all operation with FSEventStream in the same thread, where it'll
be used.

Conflicts:
	src/unix/fsevents.c
fsevents: use shared FSEventStream

It seems that number of simultaneously opened FSEventStreams is
limited on OSX (i.e. you can have only fixed number of them on
one running system), getting past through this limit will cause
`FSEventStreamCreate` to return false and write following message
to stderr:

    (CarbonCore.framework) FSEventStreamStart: register_with_server:
    ERROR: f2d_register_rpc() => (null) (-21)

To prevent this, we must use only one shared FSEventStream with a
paths for all uv_fsevent_t handles, and then filter out events for
each handle using this paths again.

See https://github.com/joyent/node/issues/5463

Conflicts:
	include/uv-private/uv-darwin.h
	src/unix/fsevents.c