PROTON-295: decoupled tracking of store entries from put/get of store entries, fixed tracking of incoming entries to start when they are returned via get rather than when they are read off of the wire

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1480445 13f79535-47bb-0310-9956-ffa450edef68
prep for QPID-229: added tests for pn_string_t; made pn_string_t handle NULL properly; switched message implementation to use new string API

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1480604 13f79535-47bb-0310-9956-ffa450edef68