reftable/table: move reading block into block reader

reftable/table: move reading block into block reader

The logic to read blocks from a reftable is scattered across both the
table and the block subsystems. Besides causing somewhat fuzzy
responsibilities, it also means that we have to awkwardly pass around
the ownership of blocks between the subsystems.

Refactor the code so that we stop passing the block when initializing a
reader, but instead by passing in the block source plus the offset at
which we're supposed to read a block. Like this, the ownership of the
block itself doesn't need to get handed over as the block reader is the
one owning the block right from the start.

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