reftable/block: avoid decoding keys when searching restart points

reftable/block: avoid decoding keys when searching restart points

When searching over restart points in a block we decode the key of each
of the records, which results in a memory allocation. This is quite
pointless though given that records it restart points will never use
prefix compression and thus store their keys verbatim in the block.

Refactor the code so that we can avoid decoding the keys, which saves us
some allocations.

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