Create separate tpool thread for async aio

Linux mutex does not ensure that mutex are given in a FIFO order, but instead allows 'fast threads' to steal the mutex, which causes other threads to starve.

This is a problem with tpool as if there is burst of aio_reads, it will create 'nproc*2' threads to handle the request even if one thread could do the job (if the block is on a very fast device or on a memory device). If the file is on a hard disk things may be even worse.

This can be seen by just starting the server on a data directory with a large 'ib_buffer_pool' file.
In this case the startup code will on my machine create 72 threads on startup to fill the
buffer pool, which is not a good idea for most system (especially desktops) as the memory
used may not be released to the operating system.

In addition the current code does not honor the variables srv_n_read_io_threads or srv_n_write_io_threads.

The suggested fix is to use a separate tpool for just async io and use a limited number of threads for this. Here is some suggested code to use as a base:

--- b/storage/innobase/srv/srv0srv.cc

+++ b/storage/innobase/srv/srv0srv.cc

@@ -580,12 +580,13 @@ static void thread_pool_thread_end()

 void srv_thread_pool_init()

 {

+  uint max_threads= srv_n_read_io_threads + srv_n_write_io_threads;

   DBUG_ASSERT(!srv_thread_pool);

 #if defined (_WIN32)

-  srv_thread_pool= tpool::create_thread_pool_win();

+  srv_thread_pool= tpool::create_thread_pool_win(1, max_threads);

 #else

-  srv_thread_pool= tpool::create_thread_pool_generic();

+  srv_thread_pool= tpool::create_thread_pool_generic(1, max_threads);

 #endif

   srv_thread_pool->set_thread_callbacks(thread_pool_thread_init,

                                         thread_pool_thread_end);