added RetryableException handling to more pieces of DistributedLock #451

There were other pieces of DistributedLock code that called findOne and
did not parse the exception thrown by nextSafe.  See 51cb556 and #413 for
more background, that change was incomplete, this change completes it.

closes #451
added NEWS about #451
Merge pull request #454 from Tokutek/distlock

added RetryableException handling to more pieces of DistributedLock #451