Version for 4.0.
It fixes two problems:
1. The cause of the bug was that we did not check the table version for
   the HANDLER ... READ commands. We did not notice when a table was
   replaced by a new one. This can happen during ALTER TABLE, REPAIR
   TABLE, and OPTIMIZE TABLE (there might be more cases). I call the fix
   for this problem "the primary bug fix".
2. mysql_ha_flush() was not always called with a locked LOCK_open.
   Though the function comment clearly said it must.
   I changed the code so that the locking is done when required. I call
   the fix for this problem "the secondary fix".