ldb_kv: Skip @ records early in a search full scan

ldb_kv: Skip @ records early in a search full scan

@ records like @IDXLIST are only available via a base search on the specific name
but the method by which they were excluded was expensive, after the unpack the
DN is exploded and ldb_match_msg_error() would reject it for failing to match the
scope.

This uses the fact that @ records have the DN=@ prefix on their TDB/LMDB key
to quickly exclude them from consideration.

Based on analysis by Garming Sam.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13893

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Apr 10 06:23:39 UTC 2019 on sn-devel-144