Removed main.h dependency from rpcserver.cpp

Rebased by @laanwj:

- update for RPC methods added since 84d13ee: setmocktime,
  invalidateblock, reconsiderblock. Only the first, setmocktime, required a change,
  the other two are thread safe.

src/rpcserver.h

@@ -19,6 +19,16 @@
 #include "json/json_spirit_utils.h"
 #include "json/json_spirit_writer_template.h"
 
+class CRPCCommand;
+
+namespace RPCServer
+{
+    void OnStarted(boost::function<void ()> slot);
+    void OnStopped(boost::function<void ()> slot);
+    void OnPreCommand(boost::function<void (const CRPCCommand&)> slot);
+    void OnPostCommand(boost::function<void (const CRPCCommand&)> slot);
+}
+
 class CBlockIndex;
 class CNetAddr;