Refactor: Live tracking of offsets as they change, so we don't need to scan for them

[astubbs]

The state for offsets is really known at runtime, but not recorded. The scan for them in PartitionMonitor#findCompletedEligibleOffsetsAndRemove isn't really required if this is implemented.

See:

• wip! feature: Continuous encoding with per partition realtime offset space allocation back pressure #46