Hopefully fix a deadlock that I've been seeing fairly often

The deadlock happens like this:
  - We get a request to add a version at the same time as a request to
    regain the lock on the subtitles

  - In the request to add the version, we SELECT the language and get
    a read lock on it.
  - In the request to regain the lock, we UPDATE the language and get
    a wrote lock on it (but are now waiting on the read lock to
    finish)
  - In the request to add the version, we UPDATE the language (because
    we are changing subtitles_complete).  This is now waiting on the
    other request's write lock -- and we now have a deadlock.

I think that's the order of events at least.  I was able to simulate
something like that and did in fact get a deadlock error.

The solution is to use SELECT FOR UPDATE to get a write lock before
doing any work.  This makes the regain-lock request wait to get its
writelock and prevents the deadlock.
Merge pull request #1802 from pculture/locking-fix

Hopefully fix a deadlock that I've been seeing fairly often