* 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