refs/reftable: batch refname availability checks

refs/reftable: batch refname availability checks

Refactor the "reftable" backend to batch the availability check for
refnames. This does not yet have an effect on performance as
`refs_verify_refnames_available()` effectively still performs the
availability check for each refname individually. But this will be
optimized in subsequent commits, where we learn to optimize some parts
of the logic when checking multiple refnames for availability.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>