Make it possible for observers to opt-in for supporting parallelizing replay or not

[einari]

By default when replaying an observer, it is put into an exclusive state of Replaying. This means it is not going to process anything for the observer and the observer is effectively suspended.

For some observers, it is ok to not be suspended for the entire observer - only the partitions being replayed.
Which means that if you were to replay the entire observer it would start off by having all partitions suspended, but as partitions finish it can start catching up and processing new events as they come.

For DevEx we could then say that by implementing the interfaces found in #1669 you basically opt-out by saying you have to be synchronous. This would be a breaking change from existing behavior.