Add support for reading 2.1 log files

Such a feature is needed to upgrade from 2.1 to 2.2 when using HA
Add a test case for upgrading from 2.1 to 2.2 in all scenarios: embedded, server, and HA
Various cleanups and small improvements
Maybe we should properly abstract over log versions, like the LogEntryParsers are. "Current" and "some arbitrary legacy version" seem a bit vague.