Summary:
When we delete drafts, they can sometimes come back because we receive deltas about them,
find that they're not in the cache, and re-save them.
This diff fixes that by permanently incrementing the optimistic change count, which was also do
to prevent deltas from changing threads while we're in the middle of a request that will change
them further. (This may not actually be necessary anymore, because we'd ignore the delta anyway
due to it's out-of-date version?)
Test Plan: Run a couple new tests
Reviewers: drew, juan
Reviewed By: juan
Differential Revision: https://phab.nylas.com/D2665