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.

include/maxscale/worker.h

@@ -70,12 +70,17 @@ MXS_WORKER* mxs_worker_get(int worker_id);
  */
 int mxs_worker_id(MXS_WORKER* pWorker);
 
+/**
+ * Return the current worker.
+ *
+ * @return A worker, or NULL if there is no current worker.
+ */
+MXS_WORKER* mxs_worker_get_current();
+
 /**
  * Return the id of the worker.
  *
- * @return The id of the worker.
- *
- * @attention If there is no current worker, then -1 will be returned.
+ * @return The id of the worker, or -1 if there is no current worker.
  */
 int mxs_worker_get_current_id();
 
@@ -117,32 +122,6 @@ bool mxs_worker_post_message(MXS_WORKER* worker, uint32_t msg_id, intptr_t arg1,
  */
 size_t mxs_worker_broadcast_message(uint32_t msg_id, intptr_t arg1, intptr_t arg2);
 
-/**
- * Add a session to the current worker's session container. Currently only
- * required for some special commands e.g. "KILL <process_id>" to work.
- *
- * @param session Session to add.
- * @return true if successful, false if id already existed in map.
- */
-bool mxs_worker_register_session(MXS_SESSION* session);
-
-/**
- * Remove a session from the current worker's session container. Does not actually
- * remove anything from an epoll-set or affect the session in any way.
- *
- * @param id Which id to remove.
- * @return The removed session or NULL if not found.
- */
-bool mxs_worker_deregister_session(uint64_t id);
-
-/**
- * Find a session in the current worker's session container.
- *
- * @param id Which id to find.
- * @return The found session or NULL if not found.
- */
-MXS_SESSION* mxs_worker_find_session(uint64_t id);
-
 /**
  * @brief Convert a worker to JSON format
  *