reftable/block: move ownership of block reader into `struct table_iter`

reftable/block: move ownership of block reader into `struct table_iter`

The table iterator allows the caller to iterate through all records in a
reftable table. To do so it iterates through all blocks of the desired
type one by one, where for each block it creates a new block iterator
and yields all its entries.

One of the things that is somewhat confusing in this context is who owns
the block reader that is being used to read the blocks and pass them to
the block iterator. Intuitively, as the table iterator is responsible
for iterating through the blocks, one would assume that this iterator is
also responsible for managing the lifecycle of the reader. And while it
somewhat is, the block reader is ultimately stored inside of the block
iterator.

Refactor the code such that the block reader is instead fully managed by
the table iterator. Instead of passing the reader to the block iterator,
we now only end up passing the block data to it. Despite clearing up the
lifecycle of the reader, it will also allow for better reuse of the
reader in subsequent patches.

The following benchmark prints a single matching ref out of 1 million
refs. Before:

  HEAP SUMMARY:
      in use at exit: 13,603 bytes in 125 blocks
    total heap usage: 6,607 allocs, 6,482 frees, 509,635 bytes allocated

After:

  HEAP SUMMARY:
      in use at exit: 13,603 bytes in 125 blocks
    total heap usage: 7,235 allocs, 7,110 frees, 301,481 bytes allocated

Note that while there are more allocation and free calls now, the
overall number of bytes allocated is significantly lower. The number of
allocations will be reduced significantly by the next patch though.

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