couch changes deadline

Currently when using changes with a filter it can take a very long time before a change is received which makes it difficult to create checkpoints to start from in case the change feed is prematurely restarted.

e.g. if you have a huge database with and a very specific change feed selector it can take 8 hours before the first change arrives. If the service has a tendency (for whatever reason) to restart once an hour it will never be able to make progress since no checkpoint ever occurs.

It would be nice if "normal" change feeds would have an option that says "if x duration has passed" then end the change feed and return the last progress equence number.

No files or tests are named. Start by locating the changes-feed implementation and its filter handling, then review the current API behavior; done should be a specified, tested option that ends a filtered feed after the configured duration and returns the last progress sequence.

Written by the indexing model from the issue text.