Improve robustness of cluster version bookkeeping

Summary:
Ref T4292. Small fixes:

• There was a bug with the first write, where we'd write 1 but expect 0. Fix this.
• Narrow the window where we hold the isWriting lock: we don't need to wait for the client to finish.
• Release the lock even if something throws.
• Use a more useful variable name.

Test Plan:

• Made new writes to a fresh cluster repository.
• Made sequential writes.
• Made concurrent writes.
• Made good writes and bad writes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4292

Differential Revision: https://secure.phabricator.com/D15747