make getSharedInstance take an optional config
make getSharedInstance take an optional config

Summary:
Previously, `getSharedInstance` does not provide a way to config the watchman
instance. This diff adds a way. If the config mismatch, then different
instances will be used.

Unused hasSharedInstance and setSharedInstance are removed.

Reviewed By: genevievehelsel

Differential Revision: D38065095

fbshipit-source-id: b52d5db407714f5c90a22fe900e127f286895d43

facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman