reftable/record: extract function to decode key lengths

reftable/record: extract function to decode key lengths

We're about to refactor the binary search over restart points so that it
does not need to fully decode the record keys anymore. To do so we will
need to decode the record key lengths, which is non-trivial logic.

Extract the logic to decode these lengths from `refatble_decode_key()`
so that we can reuse it.

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