THD specifics for plugins

This patch adds generic infrastructure for plugins to store
things on a THD object (rather that polluting THD object with
new member variables)

The feature is modeled after pthread specifics hence
the plugin shall at startup create a key, that can later be used
by multiple thread in parallel wo/ locks (if access is made to different
THD objects). The creation/destruction of a key is though mutex protected.
The feature is similar to ha_data, but exists for any type of plugin.