[RPC] remove the option of having multiple timer interfaces

src/httprpc.cpp

@@ -231,7 +231,7 @@ bool StartHTTPRPC()
 
     assert(EventBase());
     httpRPCTimerInterface = new HTTPRPCTimerInterface(EventBase());
-    RPCRegisterTimerInterface(httpRPCTimerInterface);
+    RPCSetTimerInterface(httpRPCTimerInterface);
     return true;
 }
 
@@ -245,7 +245,7 @@ void StopHTTPRPC()
     LogPrint("rpc", "Stopping HTTP RPC server\n");
     UnregisterHTTPHandler("/", true);
     if (httpRPCTimerInterface) {
-        RPCUnregisterTimerInterface(httpRPCTimerInterface);
+        RPCUnsetTimerInterface(httpRPCTimerInterface);
         delete httpRPCTimerInterface;
         httpRPCTimerInterface = 0;
     }