Old deletes need to be purged during Compaction

[weggert]

Before 'live' compaction, DELETE events older than 7 days (by default) were compacted out during WalTool compaction.

When Sirius switched to using live compaction, that bit of logic was left out. So if a DELETE event is not followed by a subsequent PUT then it lives in the Uberstore forever.

For our usage, this has led to more than 165,000,000 stale DELETE events in the Uberstore. That represents almost 3x the live PUTs and 15gb out of the 40gb Uberstore.

Sirius needs to re-add the ability to purge DELETE events that are older than a specified age while doing compaction.

[clinedome-work]

Thanks for the issue and the immediate PR. Reviewing now.