Merge pull request #1908 from spicyj/simpler-server-willmount

Simpler way to prevent flushing updates on server
Add ReactUpdates.setImmediate for async callbacks

Callbacks passed to this setImmediate function are called at the end of the current update cycle, which is guaranteed to be asynchronous but in the same event loop (with the default batching strategy).

This is useful for new-style refs (#1373, #1554) and for fixing #1698.

Test Plan: jest