lib-storage: Added "oldestonly" search arg to stop searching after the first non...

lib-storage: Added "oldestonly" search arg to stop searching after the first non-match.
This parameter works only for doveadm search queries. It's not fully exact
currently, because if mailbox.search_next_update_seq() skips over
non-matching messages we don't stop if the next message matches. So this
parameter is mainly useful for optimization of commands like:

doveadm expunge -u user@domain mailbox inbox savedsince 30d oldestonly

Where the timestamps should be ascending all the time anyway and there's no
point in continuing to search for more mails after the first timestamp is
too high.