Keep status on lock.

Every now and then in production, our status file is corrupted.  I have
a *hunch* that this is due to multiple threads of a single fedmsg-hub
trying to write their status at the same time.

This locking should ensure that only one at a time gets to do it.
Merge pull request #270 from fedora-infra/feature/keep-status-on-lock

Keep status on lock.