Add lifetimes

Add another explicit lifetime for `xcb_parts[0].iov_base`. This is the only other place (I think) we use `ffi.new()` and need to hold on to pointers explicitly, and all our uses of `ffi.cast()` look like they shouldn't suffer from having the memory deleted out from under them.

Also, re-enable the `@ensure_connected` decorator on send request. I'm able to run qtile on this xcffib with the decorator, and all the tests can pass (!!!!!) (https://travis-ci.org/flacjacket/qtile/builds/32488003). Maybe we should have roger try it out, too.