Parallelize Perforce diff generation
This change tweaks PerforceClient.diff to delegate diff generation to a

thread pool.  This is motivated by the fact that Perforce diff generation

involves roundtrips to a Perforce server, and can be quite slow when

working with large changesets.
Resolves bug 4806.

nosetests rbtools/clients/tests/test_p4.py
Before this change, posting a review update for a change with 179 deleted

  files took over four minutes.  After the change, my wait time is down to

  55s.