Support quiet mode (smaller log frequency).
Introduce sticky commit numbers.

After a view changes, replicas must effectively truncate their
logs back to their commit numbers (the primary may change log
entries past the commit number). Rather than truncate the log
directly, though, we mark the commit number as sticky, and only
acknowledge that number, until the primary enlightens us about the
later log state.

Also some readability improvements.
Improved check actually verifies replication, not just replication counts.