source4 samr: cache samr_QueryDisplayInfo results

source4 samr: cache samr_QueryDisplayInfo results

Add a cache of GUID's that matched the last samr_QueryDisplayInfo made on a
domain handle.  The cache is cleared if the requested start index is
zero, or if the level does not match that in the cache.

The cache is maintained in the guid_caches array of the dcesrv_handle.

Note: that currently this cache exists for the lifetime of the RPC
      handle.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>