s4-ldbwrap: added re-use of ldb contexts in ldb_wrap_connect()

This allows us to reuse a ldb context if it is open twice, instead
of going through the expensive process of a full ldb open. We can
reuse it if all of the parameters are the same.

The change relies on callers using talloc_unlink() or free of a parent
to close a ldb context.

source4/ldap_server/ldap_server.c

@@ -548,7 +548,7 @@ static NTSTATUS add_socket(struct tevent_context *event_context,
 
 	/* And once we are bound, free the tempoary ldb, it will
 	 * connect again on each incoming LDAP connection */
-	talloc_free(ldb);
+	talloc_unlink(ldap_service, ldb);
 
 	return status;
 }