PROTON-200: allow recv(-1) to grant unlimited credit

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1448216 13f79535-47bb-0310-9956-ffa450edef68
PROTON-200: make similar changes to Java MessengerImpl

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1448218 13f79535-47bb-0310-9956-ffa450edef68
PROTON-200: maintain a minimum credit level for each receive link

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1448220 13f79535-47bb-0310-9956-ffa450edef68