* Go: In httpBatchStore, order outgoing Chunks by ref-height
Rather than sending chunks to the server in the order that callers put
them into an httpBatchStore, instead order them by ref-height at the
time that Flush() is called. Making this change requires that
ref-height for a given chunk be passed in through the SchedulePut()
API, because Chunks (by design) don't carry much metadata about
themselves.
Toward #1510
* Go: Back datas.unwrittenPutCache with a LevelDB
httpBatchStore was caching as-yet-unwritten Chunks both in memory and
on disk. To avoid this, the in-memory cache is now backed by a
LevelDB, which handles spilling to disk when it needs to. When it's
time to send Chunks to the server, the cache is enumerated in
insert-order so that the payload of the write request is properly
structured.
Fixes#1348
cmasone-attic