WIP: Allow the adding of any fds to the poll set

This is just a first step in a trial that will allow the addition
of any file descriptor to the general poll mechanism and hence
allow any i/o to be handled by the worker threads.

There is a structure

  typedef struct mxs_poll_data
  {
      void (*handler)(struct mxs_poll_data *data, int wid, uint32_t events);
      struct
      {
          int id;
      } thread;
  } MXS_POLL_DATA;

that any other structure (e.g. a DCB) encapsulating a file descriptor must
have as its first member (a C++ struct could basically derive from it).

That structure contains two members; 'handler' and 'thread.id'. Handler is a
pointer to a function taking a pointer to a struct mxs_poll_data, a worker thread
if and an epoll event mask as argument.

So, DCB is modified to have MXS_POLL_DATA as its first member and 'handler'
is initialized with a function that *knows* the passed MXS_POLL_DATA can
be downcast to a DCB.

process_pollq no longer exists, but is now called process_pollq_dcb. The
general stuff related to statistics etc. will be moved to poll_waitevents
itself after which the whole function is moved to dcb.c. At that point,
the handler pointer will be set in dcb_alloc().

Effectively poll.[h|c] will provide a generic mechanism for listening on
whatever descriptors and the dcb stuff will be part of dcb.[h|c].

include/maxscale/dcb.h

@@ -23,6 +23,7 @@
 #include <maxscale/authenticator.h>
 #include <maxscale/ssl.h>
 #include <maxscale/modinfo.h>
+#include <maxscale/poll_core.h>
 #include <netinet/in.h>
 
 MXS_BEGIN_DECLS
@@ -184,6 +185,7 @@ typedef enum
  */
 typedef struct dcb
 {
+    MXS_POLL_DATA   poll;
     skygw_chk_t     dcb_chk_top;
     bool            dcb_errhandle_called; /*< this can be called only once */
     bool            dcb_is_zombie;  /**< Whether the DCB is in the zombie list */
@@ -228,7 +230,6 @@ typedef struct dcb
     bool            was_persistent;  /**< Whether this DCB was in the persistent pool */
     struct
     {
-        int id; /**< The owning thread's ID */
         struct dcb *next; /**< Next DCB in owning thread's list */
         struct dcb *tail; /**< Last DCB in owning thread's list */
     } thread;