r17459: As by Jerry's word commit this without his review.

r17459: As by Jerry's word commit this without his review.

This patch add some missing async functions to
solve UID/GID -> SID requests not just out of the cache,
but down the remote idmap if necessary.

This patch solves the problem of servers not showing users/groups names
for allocated UID/GIDs when joined to a group of servers that share a
prepopulated idmap backend.

Also correctly resolve UID/GIDs to SIDs when looking ACLs from the
windows security tab on teh same situation.

Simo.
(This used to be commit b8578bfab6a04fcd65a2e65f507067459e326077)