Increase dt between heartbeat and inactive timeouts

The current theory is that on occasion the inactivity timeout in the
ibrowse client fires before the replicator receives a heartbeat line.
This patch replaces the 5 second delta with the "div 3" logic that
results in a 20 second delta by default.  This is the logic that we used
back when the replicator used heartbeats instead of timeouts to control
the frequency of heartbeat lines.

As an aside, the timeout value we're adjusting here actually serves a
dual purpose.  When a _changes response is streaming this parameter
controls the frequency of heartbeat newlines, useful e.g. for highly
selective filtered feeds.  In between streams of _changes it controls
how long the _changes coordinator will wait for updates to the DB before
terminating the response with a last_seq entry.

BugzID: 17709
Merge pull request #28 from cloudant/17709-avoid-inactivity-timeout

Increase dt between heartbeat and inactive timeouts

BugzID: 17709