PROTON-139: provide access to distribution mode in source terminus

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1499758 13f79535-47bb-0310-9956-ffa450edef68
PROTON-81: provide accessors to link settlement mode

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1500080 13f79535-47bb-0310-9956-ffa450edef68