Scalability improvements

• Add EEXIST exception to the interface
• Allow Ocaml xenstore clients to receive oversized replies from xenstored
• Return the task for a wait asynchronously
• Increase maximum incoming watch limit from 1024 to 65536
• Don't leak watch strings in clients
• Add opam file
• Ensure errors from wait functions aren't lost
• Fix a non-tail call in the dispatcher
• Switch to ppx from camlp4