Cache.stop() should wait for on-going txs to finish

At first glance, this should be relatively easy to achieve by making sure TransactionTable.stop() waits for local and remote transaction collections to be empty. Clearly, waiting for txs to finish would be configurable with a timeout. This timeout should not be bigger than the transaction timeout, otherwise you're wasting time cos even if a transaction started just before Cache.stop() was called, it could only last as long as the TM says.