We were seeing serialization errors when taking out multiple read locks.
The transactions were retried, so isn't causing any failures.
Introduced in #15782.
* Make lock better handle process being killed
If the process gets killed and restarted (so that it didn't have a
chance to drop its locks gracefully) then there may still be locks in
the DB that are for the same instance that haven't yet timed out but are
safe to delete.
We handle this case by a) checking if the current instance already has
taken out the lock, and b) if not then ignoring locks that are for the
same instance.
* Periodically check for old staged events
This is to protect against other instances dying and their locks timing
out.
