Assert that keys don't point to bitmap pages.

storage/maria/ma_blockrec.h:
  assertion
storage/maria/ma_locking.c:
  With concurrent INSERTs, it is possible that two threads enter _ma_mark_file_changed()
  at the same time, so they should serialize their access to the "changed" state/share
  members; another reason is that this function may call _ma_update_state_lsns_sub()
  which may call translog_deassign_id_from_share() (I saw it during testing of
  online backup) which requires the intern_lock mutex.
  As INSERTs only change from "not changed" to "changed", we can first check without mutex:
  if it says "changed", some other thread has set or is setting the variables now,
  we don't need to do it; if it says "not changed", we serialize and re-check.