Advance the feature of storagemgr log consumer

This MR is extracted from !7134 (closed).

This MR concentrates on allowing the consumer to read a particular log entry at the time they consume. We clean up log entries when they fail behind the low water mark. The low water mark is the minimum LSN acknowledged by the log consumer and the lowest LSN that is still referred to by inflight transactions. Thus, at the time the consumer reads the entry, the target log entry should exist.