smbd: Restructure brl_get_locks_readonly

smbd: Restructure brl_get_locks_readonly

This is step 1 to get rid of brl_get_locks_internal with its complex readonly
business. It also optimizes 2 things: First, it uses dbwrap_parse_record to
avoid a talloc and memcpy, and second it uses talloc_pooled_object.

And -- hopefully it is easier to understand the caching logic with
fsp->brlock_rec and the clustering escape.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>