Add some basic bandwidth counters

• A new BandwidthCounter singleton inside onion-tunnel provides the ability to increment some atomic integers, and exposes this outside the crate for consumers to query.
• We use the TCP wrapper we pass to Arti to count how much traffic it sends and receives, as an initial set of bandwidth stats.
• This is then exposed through the JNI to Java code.
• (necessitating fixing up the panic handler so it works properly when you actually return something)

There's also a really bad Android UI to prod at this, which you might not want to merge...