PROTON-696: Rearrange the header files:
- Reduce transitive header inclusions
- Remove proton/util.h and move its contents to src/util.h. All
  the defined symbols were internal.
- Stop including with lines like #include "../blah.h" as this
  isn't needed anymore.
(Fixes windows breakage from PROTON-676)

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1627219 13f79535-47bb-0310-9956-ffa450edef68