Package should prefer _selector filters over JavaScript filter functions

[willholley]

Cloudant / CouchDB has supported using Query selectors as an alternative to JavaScript filter functions for over a year. Query selectors have a significant performance advantage over using JavaScript filters - they can be roughly an order of magnitude more efficient - and do not require users to inject a design document prior to reading changes.

The OpenWhisk package should strongly push users towards using Query selectors in favour of JS functions. Essentially the changes would be:

• use filter=_selector instead of asking users to specify a design doc/filter
• make a POST request to _changes instead of GET
• in the request body, include a "selector" field containing the Query selector to match documents against.