commitlog: Move cycle/sync/flush logic to message loop

Makes all write/flush operations into messages processing
controlled/serialized by a (secondary) task loop.

Actual IO becomes serialized, and should be processed in
the order we work through segments.

This includes periodic sync, even though we probably want to
nice it up a bit once replenish/new/delete moves as well.
Should be cleaned up to handle close/terminate in yet a
separate loop, as to not block active segment writes.
This includes flushing a discarded segment.

The places we actually need to wait for things, we use the
waitable counters. Note that timed/batch cycle right now
_issues_ the write/flush without actual waiting, the only
delay is through actual serialization of operations and
timeout is used for the full op-wait. In later stages we
probably want to change it so messages are delayed in some
cases, causing latencies, but limiting IO/disk usage.