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/poll_core.h

@@ -0,0 +1,83 @@
+#pragma once
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl.
+ *
+ * Change Date: 2019-07-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+/**
+ * @file poll_basic.h  The Descriptor Control Block
+ */
+
+#include <maxscale/cdefs.h>
+
+typedef struct mxs_poll_data
+{
+    /** Pointer to function that knows how to handle events for this particular
+     *  'struct mxs_poll_data' structure.
+     *
+     * @param data    The `mxs_poll_data` instance that contained this pointer.
+     * @param wid     The worker thread id.
+     * @param events  The epoll events.
+     */
+    void (*handler)(struct mxs_poll_data *data, int wid, uint32_t events);
+
+    struct
+    {
+        /**
+         * The id of the worker thread
+         */
+        int id;
+    } thread;
+} MXS_POLL_DATA;
+
+/**
+ * A file descriptor should be added to the poll set of all workers.
+ */
+#define MXS_WORKER_ALL -1
+
+/**
+ * A file descriptor should be added to the poll set of some worker.
+ */
+#define MXS_WORKER_ANY -2
+
+/**
+ * Add a file descriptor with associated data to the poll set.
+ *
+ * @param wid      `MXS_WORKER_ALL` if the file descriptor should be added to the
+ *                 poll set of all workers, `MXS_WORKER_ANY` if the file descriptor
+ *                 should be added to some worker, otherwise the id of a worker.
+ * @param fd       The file descriptor to be added.
+ * @param events   Mask of epoll event types.
+ * @param data     The structure containing the file descriptor to be
+ *                 added.
+ *
+ *                 data->handler  : Handler that knows how to deal with events
+ *                                  for this particular type of 'struct mxs_poll_data'.
+ *                 data->thread.id: Will be updated by `poll_add_fd_to_worker`.
+ *
+ * @attention If the descriptor should be added to all workers, then the worker
+ * thread id will be 0.
+ *
+ * @return 0 on success, non-zero on failure.
+ */
+int poll_add_fd_to_worker(int wid, int fd, uint32_t events, MXS_POLL_DATA* data);
+
+
+/**
+ * Remove a file descriptor from a poll set.
+ *
+ * @param wid      `MXS_WORKER_ALL` if the file descriptor should be removed from
+ *                 the poll set of all workers; otherwise the id of a worker.
+ * @param fd       The file descriptor to be removed.
+ *
+ * @return 0 on success, non-zero on failure.
+ */
+int poll_remove_fd_from_worker(int wid, int fd);