Allow lengthy block reconnections to be interrupted

When the coin database is out of date with the block database, the
best block in it is automatically switched to. This reconnection
process can take time, so allow it to be interrupted.

This also stops block connection as soon as shutdown is requested,
leading to a faster shutdown.

src/util.cpp

@@ -64,7 +64,7 @@ bool fDebug = false;
 bool fDebugNet = false;
 bool fPrintToConsole = false;
 bool fPrintToDebugger = false;
-bool fRequestShutdown = false;
+volatile bool fRequestShutdown = false;
 bool fShutdown = false;
 bool fDaemon = false;
 bool fServer = false;