Simplify syncmanager_lock.

Summary:
Acquiring syncmanager_lock formerly meant acquiring both a gevent lock and a
file lock. But the latter is patently unnecessary, because the
Account.sync_lock() and Account.sync_unlock() methods (invoked in
inbox.mailsync.service) already guard against an account sync running on
distinct processes on the same machine.

We probably don't actually need any sort of lock most places where we hold
syncmanager_lock, but that's a patch that needs a bit more thought and care.

Test Plan: Run sync as sanity check.

Reviewers: spang, siro

Reviewed By: siro

Differential Revision: https://review.inboxapp.com/D913