reftable/block: simplify how we track restart points

reftable/block: simplify how we track restart points

Restart points record the location of reftable records that do not use
prefix compression and are used to perform a binary search inside of a
block. These restart points are encoded at the end of a block, between
the record data and the footer of a table.

The block structure contains three different variables related to these
restart points:

  - The block length contains the length of the reftable block up to the
    restart points.

  - The restart count contains the number of restart points contained in
    the block.

  - The restart bytes variable tracks where the restart point data
    begins.

Tracking all three of these variables is unnecessary though as the data
can be derived from one another: the block length without restart points
is the exact same as the offset of the restart count data, which we
already track via the `restart_bytes` data.

Refactor the code so that we track the location of restart bytes not as
a pointer, but instead as an offset. This allows us to trivially get rid
of the `block_len` variable as described above. This avoids having the
confusing `block_len` variable and allows us to do less bookkeeping
overall.

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