Add server state helper functions

Added helper functions that check various server states. This makes the
readwritesplit code easier to read as the function names convey the
intention better than the macro invokations.

server/modules/routing/readwritesplit/readwritesplit.cc

@@ -354,8 +354,8 @@ static bool reroute_stored_statement(ROUTER_CLIENT_SES *rses, const SRWBackend&
             SRWBackend& backend = *it;
 
             if (backend->in_use() && backend != old &&
-                !SERVER_IS_MASTER(backend->server()) &&
-                SERVER_IS_SLAVE(backend->server()))
+                !backend->is_master() &&
+                backend->is_slave())
             {
                 /** Found a valid candidate; a non-master slave that's in use */
                 if (backend->write(stored))