Do schedule backfills incrementally (#5344)

Previously schedule backfills were processed synchronously: the workflow
would run through the whole given time range and either start workflows
or add them to the buffer. This is changed to process them incrementally
and keep track of the time range processed so far. There can be up to
1000 buffered backfills (reusing same limit as action buffer).

- This allows backfills to be arbitrarily long, instead of limited to
1000 actions.
- In some situations, evaluating a long backfill could take more than 1s
of elapsed time, causing the SDK to think that the workflow was
deadlocked and failing the workflow task, effectively causing the
scheduler workflow to get stuck.

new unit tests, replay test