Issue 4506 - Improve SASL logging

Description:

Converted all SLAPI_LOG_TRACE logging to Connection logging (SLAPI_LOG_CONNS).

sasl_errstring() perform a simple and fast switch case mapping from
error code to const string.

relates : 389ds#4506

Signed-off-by: Christian Heimes <cheimes@redhat.com>

Reviewed by: mreynolds