Rolled back significant changes introdced in 7776465 and 123ba73. Basically, no longer using "readiness" as a check on whether or note a leader services requests from clients. Also removed the notion of a "majority consent" when processing append entries responses.

The reason for the change is that it doesn't solve the original issue: if a member lags behind the leader and doesn't commit at same time, then member may *never* get all updates, leading to inconsistencies and "blocked" members who can't advance.
Refined computation of previous term, as it was too fixed before.
Big cleanup: we weren't properly "catching up" members who had fallen behind (e.g., their commit index lower than leaders).  Now fixed, unit tests passing.