Enforce proper use of terms "purge" "dump" and "prune" in repair/replay

[AshwinSekar]

Problem

We often incorrectly use these terms interchangeably in repair and replay which causes confusion

Proposed Solution

Rename to the following convention (open to suggestion)

• Purged: Removal of slot from blockstore, usually slots < current root
• Pruned: Removal of slots > current root that are not part of the rooted path
• Dumped: Removal of duplicate slots for the purpose of repair

[behzadnouri]

imo "dump" is mostly used when offloading from memory to disk or something like that;
so the other two terms sounds more accurate to me if the slots are actually removed rather than persisted to disk.