Make sure the backtrace is logged correctly on server internal error.
Use external ocamlfind packages for extprot and ocaml-leveldb.

Allow to use custom extprot and ocaml-leveldb with EXTPROT_DIR and
LEVELDB_DIR.
Update META, install client, server, util libs as sub-packages.
Update README.md (remove references to ocaml-leveldb and extprot).