remove frequency from checkpointer.Start interface

[nordfjord]

It's a feature specific to Cosmos/Dynamo stores. Having Sql checkpointers implement the same interface causes some confusion

e.g. We will log that there's a "checkpoint event interval" even if our checkpoints aren't event based at all.

I misunderstood the concept myself and thought it was there to tell propulsion to "only commit a checkpoint at most once every interval"

[bartelink]

YOLO merging this without considering the whole problem....