When there's no concensus, we were forming a vote every 30

When there's no concensus, we were forming a vote every 30
minutes, but writing the "valid-after" line in our vote based
on our configured V3AuthVotingInterval: so unless the intervals
matched up, we immediately rejected our own vote because it didn't
start at the voting interval that caused us to construct a vote.

This caused log entries like:

Oct 23 01:16:16.303 [notice] Choosing expected valid-after time
as 2007-10-23 05:30:00: consensus_set=0, interval=1800
...
Oct 23 01:20:01.203 [notice] Choosing valid-after time in vote as
2007-10-23 06:00:00: consensus_set=0, interval=3600
Oct 23 01:20:01.290 [warn] Rejecting vote with valid-after time of
2007-10-23 06:00:00; we were expecting 2007-10-23 05:30:00
Oct 23 01:20:01.291 [warn] Couldn't store my own vote! (I told
myself, 'Bad valid-after time'.)

Nick, you should look at this, as it's your design. :)

svn:r12129