Competing views of External Compactions in new Monitor

[dlmarion]

#5012 introduced changes into the CompactionCoordinator and Monitor to store and display the top 50 longest running compactions in the cluster. #6155 ported the logic from the legacy Monitor code over to the new monitor, which will store and display all external compactions.

We'll need to decide on the best approach and remove the unused code.

If we retain the changes in #6155, then we should move that information gathering out of the Monitor class and into the InformationFetcher class, which periodically gathers information to create a SystemInformation object to provide a static view for a small amount of time.

[DomGarguilo]

Im currently working on #6090 which is related. Here is what I am thinking for a good path forward to retain improvements and also functionality that we want:

• Snapshot and store all external compactions in SystemInformation
• expose two views of that data
  • top N for the active compactions page
  • diagnostic/raw full list