Simpler way to prevent flushing updates on server

Test Plan: Only failing tests in jest are immutable tests that were already failing.
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
Use setImmediate to defer value restoration

Depends on #1758.

Fixes #1698.

Previously, controlled components would update too soon when using something like ReactLayeredComponentMixin (i.e., before the layer's updates could propagate from the parent), causing the cursor to jump even when always updating the new model value to match the DOM state. With this change, we defer the update until after all nested updates have had a chance to finish, which prevents the cursor from misbehaving.

Also cleaned up the logic around updating a bit -- the .value and .checked updates in ReactDOMInput weren't being relied on at all so I removed them and opted for a simple forceUpdate instead. I also got rid of _isChanging which hasn't been necessary since the introduction of update batching.

Test Plan: Tested the example in http://jsfiddle.net/Bobris/ZZtXn/2/ and verified that the cursor didn't jump. Changed the code to filter out numbers and verified that the field prevents typing numbers (attempting to do so still causes the cursor to jump to the end). Also verified that controlled and uncontrolled radio buttons, textareas, and select boxes work.