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_plugin_services.h

@@ -36,9 +36,23 @@ static struct thd_alloc_service_st thd_alloc_handler= {
   thd_make_lex_string
 };
 
+static struct thd_wait_service_st thd_wait_handler= {
+  thd_wait_begin,
+  thd_wait_end
+};
+
+static struct my_thread_scheduler_service my_thread_scheduler_handler= {
+  my_thread_scheduler_set,
+  my_thread_scheduler_reset,
+};
+
+
 static struct st_service_ref list_of_services[]=
 {
   { "my_snprintf_service", VERSION_my_snprintf, &my_snprintf_handler },
-  { "thd_alloc_service",   VERSION_thd_alloc,   &thd_alloc_handler }
+  { "thd_alloc_service",   VERSION_thd_alloc,   &thd_alloc_handler },
+  { "thd_wait_service",    VERSION_thd_wait,    &thd_wait_handler },
+  { "my_thread_scheduler_service",
+    VERSION_my_thread_scheduler, &my_thread_scheduler_handler },
 };