reftable/reader: separate concerns of table iter and reftable reader

reftable/reader: separate concerns of table iter and reftable reader

In "reftable/reader.c" we implement two different interfaces:

  - The reftable reader contains the logic to read reftables.

  - The table iterator is used to iterate through a single reftable read
    by the reader.

The way those two types are used in the code is somewhat confusing
though because seeking inside a table is implemented as if it was part
of the reftable reader, even though it is ultimately more of a detail
implemented by the table iterator.

Make the boundary between those two types clearer by renaming functions
that seek records in a table such that they clearly belong to the table
iterator's logic.

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