(#3195) - Stringify "since" parameter when fetching _changes

In CouchDB 2.0, the sequence number may be represented as an array.
This commit stringifies the "since" parameter if it is an object to cope
with this scenario.
(#136) - Fix "Test create attachment and doc in one go without callback" for clustered CouchDB

We cannot rely on an ordered update sequence in clustered CouchDB.
Instead of looking at the sequence number, identify a change using
the document id.
(#136) - Fix "Test create attachment without callback" against clustered CouchDB

We cannot rely on an ordered update sequence in clustered CouchDB.
Instead, request the _changes feed following the initial docment update
using the "since" parameter. There is still potential for this to fail
because _changes may return entries before that specified by "since" -
the guarantee is that all entries following "since" will be returned.

Possibly one to review if we find that the test fails occasionally
(for me it is stable).
(#136) - Skip attachments tests which fail due to CouchDB 2.0 bugs

Skip attachment tests which fail due to missing support for attachments=true
in the CouchDB 2.0 changes feed
(https://issues.apache.org/jira/browse/COUCHDB-2519)