Use reflected BT address if we don't know our own address

This branch allows the BT plugin to use a local address learned from reflected transport properties if the OS doesn't allow us to know the local address directly.

Once we've made an outgoing BT connection to any contact, the contact should reflect our address back to us. We can then share the address with our other contacts, allowing them to connect to us.

We accept a reflected address if the same address is reported by the majority of contacts that reflect an address. Reflected addresses are ignored if the OS allows us to know the local address directly.