Added insert-intervals-unit for sub-second granularity

[MoellJ]

With the insert-intervals parameter one can control how frequently a batch is inserted into the DB by a worker. If one worker should insert multiple batches per second while still being rate-limited we need sub-second insert-intervals which currently isn't possible.

This PR fixes that by introducing millisecond and microsecond granularity to insert-intervals via insert-intervals-unit.