Convert listing exports to query2

This solves one case where locks are acquired during LUXI queries.
Pretty late into the transition I noticed that OpBackupQuery had a
“use_locking” parameter for a long time, but didn't use it. Since
most of the other changes were already and this allows exports to
be listed via RAPI (/2/query) I decided to finish.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>