[MERGE] Change the signature on VersionedFiles adapters to allow less typing and more flexability inside adapters.

[Vincent Ladeuil]

>>>>> "robert" == Robert Collins <robertc at robertcollins.net> writes:

    robert> ``VersionedFiles`` record adapters have had their signature change
    robert> from ``(record, record.get_bytes_as(record.storage_kind))`` to
    robert> ``(record)`` reducing excess duplication and allowing adapters
    robert> to access private data in record to obtain content more
    robert> efficiently. (Robert Collins)

    robert> This reduces friction showing up in the
    robert> versionedfiles-for-network-streaming project I am working on with
    robert> Andrew.

BB:approve

        Vincent