[REST] give an appropriate response in warmup phase

src/rpcserver.h

@@ -53,6 +53,9 @@ void SetRPCWarmupStatus(const std::string& newStatus);
 /* Mark warmup as done.  RPC calls will be processed from now on.  */
 void SetRPCWarmupFinished();
 
+/* returns the current warmup state.  */
+bool RPCIsInWarmup(std::string *statusOut);
+
 /**
  * Type-check arguments; throws JSONRPCError if wrong type given. Does not check that
  * the right number of arguments are passed, just that any passed are the correct type.