Test couchdb 2.0 developer preview, including bigcouch merge

"this issue":https://issues.apache.org/jira/browse/COUCHDB-1999 is tracking the merge of the bigcouch branch into couchdb 1.6

(from redmine: created on 2013-12-29, closed on 2016-05-26, relates #2685, duplicates #5830 (closed), blocks #4887, blocks #5074)