There's a bunch of moving pieces here:
- Add a new RingWriter class.
 Stick it in a new swift.common.ring.io module. You *can* use it like
 the old gzip file, but you can also define named sections which can
 be referenced later on read. Section names may be arbitrary strings,
 but the "swift/" prefix is reserved for upstream use. Sections must
 contain a single length-value encoded BLOB. If sections are used, an
 additional BLOB is written at the end containing a JSON section-index,
 followed by an uncompressed offset for the index.
 Move RingReader to ring/io.py, too.
- Clean up some ring metadata handling:
 - Drop MD5 tracking in RingReader. It was brittle at best anyway, and
 nothing uses it. YAGNI
 - Fix size/raw_size attributes when loading only metadata.
- Add the ability to seek within RingReaders, though you need to know
 what you're doing and only seek to flush points.
- Let RingBuilder objects change how wide their replica2part2dev_id
 arrays are. Add a dev_id_bytes key to serialized ring metadata.
 dev_id_bytes may be either 2 or 4, but 4 requires v2 rings. We
 considered allowing dev_id_bytes of 1, but dropped it as unnecessary
 complexity for a niche use case.
- swift-ring-builder version subcommand added, which takes a ring. This
 lets operators see the serialization format of a ring on disk:
 $ swift-ring-builder object.ring.gz version
 object.ring.gz: Serialization version: 2 (2-byte IDs), build version: 54
Signed-off-by: Tim Burke <tim.burke@gmail.com>
Change-Id: Ia0ac4ea2006d8965d7fdb6659d355c77386adb70