This patch moves thread-specific keycache variables from the mysys
thread local storage struct to a separate struct managed by MyISAM.
This reduces thread local storage usage and is a step towards
implementing WL#6817 "Getting rid of mysql_thread_end() and thread
local storage in libmysqlclient".

The mysys TLS condition variable was used by both the keycache and
thr_lock. The thr_lock usage is now handled by a new, separate
condition variable owned by THD.

The keycache condition variable is now managed by MyISAM code and
Ha_data::ha_ptr is used to manage a reference to it.