After this patch, my_thread_init() will no longer allocate
memory for the mysys thread local variables struct for release builds.
This means that not being able to call my_thread_end() will no
longer lead to memory leak.

After several earlier patches, only two members remained in the
mysys thread local struct - both ints related to error handling.
These are now put into separate thread local entries.

With this patch, the problem which lead to the creation of
WL#6817 "Getting rid of mysql_thread_end() and thread local
storage in libmysqlclient" has been fixed.