Gh238 streaming list buckets 1.4

Adds the 1.4 feature of streaming a list buckets request. This is only supported on Riak 1.4+

It also fixes a long standing problem where your could not close/cancel a streaming list keys request.

addresses #238 and #231 

**Do not merge into master until after v1.1.2 has been cut** 

Also note to currently test this you must check out then do a `mvn clean install` on `riak_pb` as it references the current master (SNAPSHOT). Will update the `pom.xml` in this PR when that changes.