(#136) - Ignore "changes last seq" against CouchDB 2.0

In clusterd CouchDB, the sequence number returned to the client is an
encoded representation of the various shard states. Whilst the last
"seq" value in _changes.results and the "last_seq" field should evaluate
to the same thing on the server, their encoded representation may differ.
Therefore, the assertions in this test don't hold for clustered CouchDB.
(#136) - Ignore tests for #3136 against CouchDB 2.0

These appear to be testing the internal PouchDB implementation of _changes.
Many of the assumptions don't hold against clustered CouchDB so skipping
them for now.
(#136) - Fix "changes limit and view instead of filter" against CouchDB 2.0

Do not assume incremental sequence numbers in the test.
This commit also adds a helper function to dig out specific items from
_changes (seems the simplest possible solution rather than importing
lodash or similar).
Merge pull request #3249 from willholley/136-changes-limit-and-view-instead-of-filter

(#136) - Fix "changes limit and view instead of filter" against CouchDB 2.0
(#136) - Fix "#3136 style=all_docs" against CouchDB 2.0

The _changes feed does not guarantee order so remove this assertion.