reftable/block: refactor binary search over restart points

reftable/block: refactor binary search over restart points

When seeking a record in our block reader we perform a binary search
over the block's restart points so that we don't have to do a linear
scan over the whole block. The logic to do so is quite intricate though,
which makes it hard to understand.

Improve documentation and rename some of the functions and variables so
that the code becomes easier to understand overall. This refactoring
should not result in any change in behaviour.

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