Stash updates when Fast Refresh is off

Summary:
If you change a file while Fast Refresh is off, this now stashes an update instead of ignoring it. When/if you turn it on, we will apply those stash updates. This solves the confusion that can happen when you enable it midway after making a bunch of changes, and therefore makes the experience more reliable.

**This current implementation is unfortunate because the app memory load increases with the number of file saves. This isn't really sustainable. So in the next diff I will change it to only remember the *latest versions* of every edited file instead.**

Reviewed By: cpojer

Differential Revision: D16344332

fbshipit-source-id: 69609a00eb9022f6b2797269fa091fa1b4125dd1