WL#5363: Thread Pool Service Interface

In order to allow thread schedulers to be dynamically loaded,
it is necessary to make the following changes to the server:

- Two new service interfaces

- Modifications to InnoDB to inform the thread scheduler of state changes.

- Changes to the VIO subsystem for checking if data is available on a socket.

- Elimination of remains of the old thread pool implementation.

The two new service interfaces introduces are:

my_thread_scheduler
  A service interface to register a thread
  scheduler.

thd_wait
  A service interface to inform thread scheduler
  that the thread is about to start waiting.

In addition, the patch adds code that:

- Add a call to thd_wait for table locks in mysys
  thd_lock.c by introducing a set function that
  can be used to set a callback to be used when
  waiting on a lock and resuming from waiting.

- Calling the mysys set function from the server
  to set the callbacks correctly.

sql/sql_class.h

@@ -1812,6 +1812,10 @@ public:
         xid_state.xid.null();
       free_root(&mem_root,MYF(MY_KEEP_PREALLOC));
     }
+    my_bool is_active()
+    {
+      return (all.ha_list != NULL);
+    }
     st_transactions()
     {
       bzero((char*)this, sizeof(*this));
@@ -2734,13 +2738,14 @@ public:
   virtual void set_statement(Statement *stmt);
 
   /**
-    Assign a new value to thd->query and thd->query_id.
+    Assign a new value to thd->query and thd->query_id and mysys_var.
     Protected with LOCK_thd_data mutex.
   */
   void set_query(char *query_arg, uint32 query_length_arg);
   void set_query_and_id(char *query_arg, uint32 query_length_arg,
                         query_id_t new_query_id);
   void set_query_id(query_id_t new_query_id);
+  void set_mysys_var(struct st_my_thread_var *new_mysys_var);
   void enter_locked_tables_mode(enum_locked_tables_mode mode_arg)
   {
     DBUG_ASSERT(locked_tables_mode == LTM_NONE);