ITS#6437, slap_sl_mem_create():

Bugfix: Switching implementation stack<->pool on an old context could
        crash or leak, it ran the wrong implementation's cleanup code.
Cleanup:  Factor out identical stack/pool code.  Call
          slap_sl_mem_destroy(NULL,) instead of copying its pool code.
API change: slap_sl_mem_destroy(key=NULL,) gets a new meaning.  Does
            not affect current OpenLDAP code, it never passed NULL.