(#136) - Fix "Test pull replication with many changes"

CouchDB 2.0 represents sequence numbers as arrays which should
be treated as opaque blobs. This prevents us using simple
comparison operators on them (e.g. last_seq < current_seq).

Previously we were using a similar test to determine whether new
changes were returned in a call to _changes (if params.since <
result.last_seq, fetch more changes). This no longer holds so, instead,
fetch more changes if the last_seq returned is not equal to the since
parameter (if params.since != result.last_seq, fetch more changes).

There is a danger here of a loop because there is no guarantee that
last_seq will be consistent between successive calls to _changes in
a clustered CouchDB (rare that this happens but it's possible).