As a part of #1865, we need an in-process component responsible for collecting the workload that we'll eventually replay. This component should:

a) collect a consistent checkpoint of the database state, copying it to an external location
b) copy all sstables flushed and ingested after the checkpoint to an external location
c) copy all manifests in use after the checkpoint to an external location

All copying should be performed asynchronously, without blocking any of the Pebble background threads or Cockroach foreground threads.

This component can implement base.Cleaner to ensure that sstables are not removed from the filesystem before they can be captured.