PROTON-192: finished reducing visibility of proton-j Engine constructors and replacing calls with factories. Also introduced ProtonJXXX interfaces so contrib is referring to these rather than concrete classes

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/branches/jni-binding@1438470 13f79535-47bb-0310-9956-ffa450edef68