Add a CValidationInterface::TransactionRemovedFromMempool

This is currently unused, but will by used by wallet to cache when
transactions are in the mempool, obviating the need for calls to
mempool from CWalletTx::InMempool()

src/init.cpp

@@ -265,6 +265,7 @@ void Shutdown()
 #endif
     UnregisterAllValidationInterfaces();
     GetMainSignals().UnregisterBackgroundSignalScheduler();
+    GetMainSignals().UnregisterWithMempoolSignals(mempool);
 #ifdef ENABLE_WALLET
     CloseWallets();
 #endif
@@ -1240,6 +1241,7 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
     threadGroup.create_thread(boost::bind(&TraceThread<CScheduler::Function>, "scheduler", serviceLoop));
 
     GetMainSignals().RegisterBackgroundSignalScheduler(scheduler);
+    GetMainSignals().RegisterWithMempoolSignals(mempool);
 
     /* Start the RPC server already.  It will be started in "warmup" mode
      * and not really process calls already (but it will signify connections