back-ldap connection caching

[Howard Chu <hyc@symas.com>]

After running SLAMD against back-ldap I've noticed some problems in the 
approach - while a single load generator may send multiple requests over a 
single connection, back-ldap always creates new connections for each incoming 
Simple Bind, and leaves them available to be shared by other sessions.

Thinking about it, this usage doesn't really make a lot of sense. Any identity 
that's explicitly binding to back-ldap is necessarily going to be different 
from any other session's ID. The only sessions that it makes sense to share 
are those that were implicitly bound because they were authenticated 
elsewhere, and fell into this backend (via glue, typically) while processing 
some other request.

So I think this means we should separate out the explicitly bound connections 
from everything else. They should only live as long as their inbound slapd 
connection lives, and should only be used by ops from their inbound slapd 
connection.
--
  -- Howard Chu
  Chief Architect, Symas Corp.  http://www.symas.com
  Director, Highland Sun        http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP     http://www.openldap.org/project/