MXS-2002 Worker::execute() renamed to Worker::call()

In preparation for Worker::post() to be renamed to Worker:execute().
The concept of _posting_ will be reserved to mean the transfer of
something over the message queue to the worker for processing and
nothing else.

include/maxscale/worker.hh

@@ -771,7 +771,7 @@ public:
      *
      * @return True if function was executed on the worker
      */
-    bool execute(GenericFunction func, enum execute_mode_t mode);
+    bool call(GenericFunction func, enum execute_mode_t mode);
 
     /**
      * Post a message to a worker.