object/searchv2: Decode requested cursor and prepare the BoltDB one once

Previously, meta bucket initial seek key and key space prefix was
calculated inside the metabase `Search` method (BoltDB transaction to be
more precise). Since seek key and prefix are globally calculated from
the query and do not depend on a specific metabase, this incurred
resource costs.

This changes `ObjectService.SearchV2` server to:
 1. verify requested cursor before accessing storage components;
 2. calc meta bucket seek (init key, prefix) once.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>