dbwrap_rbt: add nested traverse protection

dbwrap_rbt: add nested traverse protection

Multiple dbwrap_traverse_read() calls are possible.

store() and delete() on a fetch locked record
are rejected during dbwrap_traverse_read().

A dbwrap_traverse() within a dbwrap_traverse_read()
behaves like a dbwrap_traverse_read().

Nested dbwrap_traverse() calls are not possible.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11375
BUG: https://bugzilla.samba.org/show_bug.cgi?id=11394

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