reftable/reader: make table iterator reseekable

reftable/reader: make table iterator reseekable

In 67ce50ba26 (Merge branch 'ps/reftable-reusable-iterator', 2024-05-30)
we have refactored the interface of reftable iterators such that they
can be reused in theory. This patch series only landed the required
changes on the interface level, but didn't yet implement the actual
logic to make iterators reusable.

As it turns out almost all of the infrastructure already does support
re-seeking. The only exception is the table iterator, which does not
reset its `is_finished` bit. Do so and add a couple of tests that verify
that we can re-seek iterators.

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