Merge branch 'ps/reftable-block-iteration-optim'

The code to iterate over reftable blocks has seen some optimization
to reduce memory allocation and deallocation.

* ps/reftable-block-iteration-optim:
  reftable/block: avoid copying block iterators on seek
  reftable/block: reuse `zstream` state on inflation
  reftable/block: open-code call to `uncompress2()`
  reftable/block: reuse uncompressed blocks
  reftable/reader: iterate to next block in place
  reftable/block: move ownership of block reader into `struct table_iter`
  reftable/block: introduce `block_reader_release()`
  reftable/block: better grouping of functions
  reftable/block: merge `block_iter_seek()` and `block_reader_seek()`
  reftable/block: rename `block_reader_start()`