StringAccum::operator<<(timeval)
View change updates.

- We are between views if f+1 of both old & new views have prepared.
  Doesn't matter if we've heard from the primary or not; either way
  we should ignore messages from the old primary (I think).
- A commit from the new primary confirms the new view if f+1 of both
  old & new views have prepared, whether or not primary was heard
  from previously.
- Document why a view message was sent.
Simplify definition of between_views().

Don't send empty log entries during view changes.