to cleanup open connections

It was possible to UNINSTALL storage engine plugin when binding
between THD object and storage engine is still active (e.g. in
the middle of transaction).

To avoid unclean deactivation (uninstall) of storage engine plugin
in the middle of transaction, additional storage engine plugin
lock is acquired by thd_set_ha_data().

If ha_data is not null and storage engine plugin was not locked
by thd_set_ha_data() in this connection before, storage engine
plugin gets locked.

If ha_data is null and storage engine plugin was locked by
thd_set_ha_data() in this connection before, storage engine
plugin lock gets released.

If handlerton::close_connection() didn't reset ha_data, server does
it immediately after calling handlerton::close_connection().

Note that this is just a framework fix, storage engines must switch
to thd_set_ha_data() from thd_ha_data() if they want to see fit.

include/mysql/plugin.h:
  As thd_{get|set}_ha_data() have some extra logic now, they
  must be implemented on server side.
include/mysql/plugin.h.pp:
  As thd_{get|set}_ha_data() have some extra logic now, they
  must be implemented on server side.
sql/handler.cc:
  Make sure ha_data is reset and ha_data lock is released.
sql/handler.h:
  hton is not supposed to be updated by ha_lock_engine(),
  make it const.
sql/sql_class.cc:
  As thd_{get|set}_ha_data() have some extra logic now, they
  must be implemented on server side.
sql/sql_class.h:
  Added ha_data lock.