Scale CONSENSUS_MIN_SECONDS_BEFORE_CACHING by voting interval

Scale CONSENSUS_MIN_SECONDS_BEFORE_CACHING by voting interval

If the voting interval was short enough, the two-minutes delay
of CONSENSUS_MIN_SECONDS_BEFORE_CACHING would confuse bridges
to the point where they would assert before downloading a consensus.
It it was even shorter (<4 minutes, I think), caches would
assert too.  This patch fixes that by having replacing the
two-minutes value with MIN(2 minutes, interval/16).

Bugfix for 1141; the cache bug could occur since 0.2.0.8-alpha, so
I'm calling this a bugfix on that.  Robert Hogan diagnosed this.
Done as a patch against maint-0.2.1, since it makes it hard to
run some kinds of testing networks.