WIP members now tracked in RaftState, and method added to Log so that its possible to stop committing entries, in case its not yet possible to do so.  This is groundwork for correctly handling cluster membership changes,  as it should not be possible to commit a new config change if a joint config is still in effect.  Further, the rules of Raft require that it is not safe for a leader to commit any entries until at least 1 entry from the term has been appended to a majority of members.  These conditions are not yet checked.
Slightly better checks before committing at the leader: can't commit unless the safe term (e.g., the lowest appended term on a majority of members) is equal to the leader's term. That meets the criterion specified in the Raft algorithm: at least 1 entry from the leader's term must be on a majority of members before committing.