ldb_tdb: Add ltdb_idx_to_key() and use it in ltdb_index_filter()

ldb_tdb: Add ltdb_idx_to_key() and use it in ltdb_index_filter()

This will allow a common point to parse index records into a TDB key,
allowing them to be a GUID or DN in the future

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>