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.

include/mysql/plugin.h

@@ -71,7 +71,7 @@ typedef struct st_mysql_xid MYSQL_XID;
   Plugin API. Common for all plugin types.
 */
 
-#define MYSQL_PLUGIN_INTERFACE_VERSION 0x0101
+#define MYSQL_PLUGIN_INTERFACE_VERSION 0x0102
 
 /*
   The allowable types of plugins