[AMQP Python] investigate pickling clients/transports/etc for standalone lib

[swathipil]

Original attempt at pickling pyamqp clients/transport here: 27aefbb

Need to investigate whether it's possible to pickle clients, without just removing the "connection" components in the getstate and relying on the retry loop to re-create the underlying socket/connection.

As per Anna: "[It] feels like this should be more thoroughly designed then simply stripping out anything "problematic" and relying on a retry loop to put it back together again."