PROTON-200: modified messenger's credit distribution algorithm to cope with credit scarce scenarios

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1532452 13f79535-47bb-0310-9956-ffa450edef68
PROTON-440: removed accidental println

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1532453 13f79535-47bb-0310-9956-ffa450edef68