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