- use the TimestampSecs newtype instead of raw i64s
- use FixedOffset instead of a minutes_west offset
- check localOffset each time the timing is calculated, and set it
if it's stale - even for v1.
- check for and fix missing rollover when calculating timing
- stop explicitly passing localOffset in the sync/start call
We need to be a bit conservative here due to buffer bloat - in the
upload case on a slow link, it can appear that no I/O is happening
when a buffer is draining.
Also:
- provide a way for the progress handler to skip the throttling so that
we can ensure progress is updated at the end of a stage
- show 'checking' at the end of full sync
