MXS-1915 Remove id from mxs::Worker

The id has now been moved from mxs::Worker to mxs::RoutingWorker
and the implications are felt in many places.

The primary need for the id was to be able to access worker specfic
data, maintained outside of a routing worker, when given a worker
(the id is used to index into an array). Slightly related to that
was the need to be able to iterate over all workers. That obviously
implies some kind of collection.

That causes all sorts of issues if there is a need for being able
to create and destroy a worker at runtime. With the id removed from
mxs::Worker all those issues are gone, and its perfectly ok to create
and destory mxs::Workers as needed.

Further, while there is a need to broadcast a particular message to
all _routing_ workers, it hardly makes sense to broadcast a particular
message too _all_ workers. Consequently, only routing workers are kept
in a collection and all static member functions dealing with all
workers (e.g. broadcast) have now been moved to mxs::RoutingWorker.

Now, instead of passing the id around we instead deal directly
with the worker pointer. Later the data in all those external arrays
will be moved into mxs::[Worker|RoutingWorker] so that worker related
data is maintained in exactly one place.

server/modules/protocol/MySQL/mariadb_client.cc

@@ -230,7 +230,7 @@ void LocalClient::drain_queue()
     }
 }
 
-uint32_t LocalClient::poll_handler(struct mxs_poll_data* data, int wid, uint32_t events)
+uint32_t LocalClient::poll_handler(struct mxs_poll_data* data, void* worker, uint32_t events)
 {
     LocalClient* client = static_cast<LocalClient*>(data);
     client->process(events);