reftable: add blocksource, an abstraction for random access reads

The reftable format is usually used with files for storage. However, we abstract
away this using the blocksource data structure. This has two advantages:

* log blocks are zlib compressed, and handling them is simplified if we can
  discard byte segments from within the block layer.

* for unittests, it is useful to read and write in-memory. The blocksource
  allows us to abstract the data away from on-disk files.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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