Limit the duration of a continuous _changes feed

This patch enables fabric to terminate long-running continuous or
longpoll _changes feeds if the accumulated response time has exceeded a
configurable value.  When that happens fabric will execute the callback
function with {stop, LastSeq} at the next opportunity where it would
normally send a heartbeat.

[fabric]
changes_duration = 300000

BugzID: 17371
Merge pull request #72 from cloudant/17371-disconnect-changes-eventually

Limit the duration of a continuous _changes feed

BugzID: 17371