include/waiting_threads.h:
  C_MODE_START/END
mysys/waiting_threads.c:
  relax the checks - auto init thd in will_wait_for,
  allow to destroy uninited thd (=noop),
  allow a "release" an unexistent resource (=noop),
sql/sql_class.cc:
  move wt* maintainance from maria to the server.
  do THD::cleanup after ha_close_connection() and plugin_thdvar_cleanup().
storage/maria/unittest/trnman-t.c:
  update to new prototype