Interrupt reaper thread

It is possible for the reaper thread to become blocked when telling a transaction to rollback. We should have another thread that interrupts the reaper thread if it has been blocked for more than a configurable amount of time on the same transaction. In this case, the reaper could either put the transaction back on the list and try again later, or just mark it as rollback_only. My preference would actually be to have a separate list of blocked transactions that are marked as rollback_only and have the reaper try to roll them back again later.