Issue a timeout even if messages have arrived

When processing _changes feeds a timeout message simply generates a
newline in the response body.  We can afford to send a few extra of
those.  This patch allows us to send messages from the workers back to
the coordinator without interfering with the timeout calculation.

BugzID: 17709
Handle `no_pass` messages from filtered docs

BugzID: 17709
Send a `no_pass` message when a doc fails a filter

BugzID: 17709
Merge pull request #87 from cloudant/17709-report-progress-selective-filters

Report progress as filters supress documents

BugzID: 17709