Added delay when removing tunnel info, fixed mid encoding

[devos50]

In this PR, I've added an optional delay when removing/destroying circuits, relays and exit sockets. This allows some post-mortem data to flow over the circuit, like payout messages in Tribler.

I've also addressed mid encoding in the Tunnel object. The way we encode it is a common source of errors. We now always store the mid in binary. We should eventually fix it to store a Peer object instead but requires takes a bit more work.

[qstokkink]

Question: what is the expected behavior on unload()?

[devos50]

@qstokkink good question, I didn't consider the situation when unloading an overlay. In that case, we would like to remove the data immediately I guess? This could be realized by adding a remove_now keyword argument to remove_relay, remove_circuit etc and if it's true, we remove the data immediately. What do you think about this?

[qstokkink]

@devos50 considering the payout (actual money) happens after circuit destruction, I think the entire community should wait until circuits have been destructed and have been paid out. Although it would take up to the remove_tunnel_delay for the community to unload (which is a relatively long time).

However, considering this functionality supports your Market, I'm fine with whatever you think is best.

[devos50]

@qstokkink I think that when a community unloads, it shouldn't wait for additional incoming messages. I know this might cause a payout not to complete but this is not the end of the world since at most 250MB is lost (the maximum amount of data that can go over a circuit).

I've updated my PR.