PROTON-583: Move a bunch of constant data structures to read only section
- Everywhere a pn_class_t is defined it is a constant structure that
  can be read only.
- There is a table in the SSL implementation that also is constant
  and can be moved to the read only section.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1596358 13f79535-47bb-0310-9956-ffa450edef68