Modifying Mersenne WAL to use ragel

Modified the current implementation of
packing and unpacking WAL log information
to use ragel and messagepack instead of
protobuf.
I believe we also need to bail out if `msgpack_unpack_next()` fails. Currently WAL record will be just silently skipped.

Something like this would look correct to me:

    retval = msgpack_unpack_next(&result, ptr, size, NULL);
    if (!retval) {
        /* fatal error, log and exit */
    }
    /* Continue with wal_msg_unpack */