PROTON-222: give messenger visibility into the amount of buffered output

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/branches/kgiusti-proton-225@1446691 13f79535-47bb-0310-9956-ffa450edef68
PROTON-222: modify new interface to return bool rather than size_t

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/branches/kgiusti-proton-225@1446692 13f79535-47bb-0310-9956-ffa450edef68
PROTON-222: add simple test reproducer

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/branches/kgiusti-proton-225@1446693 13f79535-47bb-0310-9956-ffa450edef68