One more step towards to better PDC.

One more step towards to better PDC.

This patch, from "Stefan (metze) Metzmacher" <metze@metzemix.de> implements an
LDAP connection cache.  This removes the quite silly situation where every
single passdb operation involved a new LDAP connection.

The hope is that this will give us a decent performance boost in some usrmgr
related activities, and in the sid->name/sid->uid code.

The remaining things I think are 'todo' for pdb_ldap (in the near term) are:
 - intergrate volker's next_rid patch for NUA accounts,
 - add a 'trust ldap ids' option (remove Get_Pwnam() hit on enumerations).
 - put the group mapping actually into ldap
 - Schema fixes and do utf8 conversion
 - server failover (try a second server for the rebind on fail)
 - ensure we block between an 'add' and the ldap master replicating to our
   local slave (mezte found this issue, kills domain joins)

Andrew Bartlett