Concurrency in `Assigner` should be restricted for memory only operations. Disk IO in `Assigner` should be submitted via a dedicated IO `ExecutionContext`

[simerplaha]

Compaction can be configured to be highly concurrent for all in-memory operations which also means that it is also concurrent sometimes for initialising Segment iterators (Disk IO) which should be sequential and not concurrent and submitted via dedicated IO ExecutionContext.

This is related to #330.