PROTON-558: Reduce the memory usage for the field name tables some more.
- Used a string pool and instead of keeping addresses of strings
  hold offsets into the string pool.
- This moves all the tables into rodata and only leaves the string pool
  pointer as relro.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1611404 13f79535-47bb-0310-9956-ffa450edef68