Revert backoff v4 -> v2 (rethinkdb#485)

Cluster discovery & rare connection leak fixes (rethinkdb#483)

Merge pull request rethinkdb#480 from cquon/fix_connpool_close

Verify the Pool connection is not nil before attempting to close it

Merge pull request rethinkdb#478 from rethinkdb/develop

6.1.0 release

6.0 version

Merge pull request rethinkdb#452 from rethinkdb/develop

fix noreply memory leak

Merge pull request rethinkdb#448 from rethinkdb/develop

rename package & integration test fixes

Merge pull request rethinkdb#446 from GoRethink/develop

External custom marshalers

Merge pull request rethinkdb#431 from GoRethink/develop

v4: connection refactor, some tests and opentracing.

fix typo at http opts