TransactionManager retries needlessly when an UnknownTransactionException is caught

In doPrepareX the the TransactionManager catches and retries UnknownTransactionExceptions thrown by the regions. This is pointless because regions that don't know about a transaction will never learn about it through a prepare message retry. Instead, we should catch a generic excpetion, look to see if it's an UnknownTransactionException, throw it if it is, otherwise retry.