Make sure only one thread tries to load a given entry at a time.
Bump up the DB environment's max TXNs to accomodate the new TXNs.

cleanup

clear shared connections when ldap_result fails with -1 (typically, remote server is down); fixes ITS#3217

failure in slap_op_free is possible)

re-sasl_encode() of the input.  sasl_encode() may be stateful.