(pouchdb/express-pouchdb#95) - add tests for longpoll changes

These verify that the http server's
_changes?feed=longpoll works as expected.

The tests pass before and after my fix for
express-pouchdb (pouchdb/express-pouchdb#96),
but the main difference is that you'll
notice a lot less being spewed out in the logs,
since it's actually longpolling.
Similiarly to below, changes feeds should be cancelled before the test completes