MXS-1754 Reintroduce maxscale::Worker

Worker is now the base class of all workers. It has a message
queue and can be run in a thread of its own, or in the calling
thread. Worker can not be used as such, but a concrete worker
class must be derived from it. Currently there is only one
concrete class RoutingWorker.

There is some overlapping in functionality between Worker and
RoutingWorker, as there is e.g. a need for broadcasting a
message to all routing workers, but not to other workers.

Currently other workers can not be created as the array for
holding the pointers to the workers is exactly as large as
there will be RoutingWorkers. That will be changed so that
the maximum number of threads is hardwired to some ridiculous
value such as 128. That's the first step in the path towards
a situation where the number of worker threads can be changed
at runtime.

server/core/resource.cc

@@ -34,7 +34,6 @@
 #include "internal/service.h"
 #include "internal/config_runtime.h"
 #include "internal/modules.h"
-#include "internal/worker.h"
 #include "internal/routingworker.hh"
 
 using std::list;
@@ -128,7 +127,7 @@ bool Resource::matching_variable_path(const string& path, const string& target)
             char* end;
             int id = strtol(target.c_str(), &end, 10);
 
-            if (*end == '\0' && mxs_worker_get(id))
+            if (*end == '\0' && mxs_rworker_get(id))
             {
                 rval = true;
             }