Allow to omit seq from some rows in _changes response

This is a hack to improve the throughput of _changes on databases with
large Q values.  The record-based approach for sharing config options is
a pain to extend so we're going to be dirty and use the pdict here.

BugzID: 23637
Merge pull request #117 from cloudant/23637-sample-sequence-publishes

Allow to omit seq from some rows in _changes response

BugzID: 23637