Currently we support garbage collection based on expiry of keys, via truncation from the head of the log. This has extremely low overhead, and captures the cases where there is a limit on TTL of all keys. However, there are cases where actual compaction of live keys is necessary. Thus, we need an mechanism to roll forward pages from the head, by determining liveness of keys in the pages to be expired, and reinserting live keys at the tail. Adding @gunaprsd.