reftable/reader: set up the reader when initializing table iterator

reftable/reader: set up the reader when initializing table iterator

All the seeking functions accept a `struct reftable_reader` as input
such that they can use the reader to look up the respective blocks.
Refactor the code to instead set up the reader as a member of `struct
table_iter` during initialization such that we don't have to pass the
reader on every single call.

This step is required to move seeking of records into the generic
`struct reftable_iterator` infrastructure.

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