Reverts to emulating the write concern to work around how prior to
MongoDB 3.4, operations that did writes didn't necessarily accept a
writeConcern object.
Also limits the usage of replica set connection strings to only the
write_concern_majority_passthrough.yml test suite to work around the
lack of complete support of MongoURI parsing in versions of the mongo
shell prior to MongoDB 3.4.
(cherry picked from commit 264d971842)