Add disconnect message

[LivInTheLookingGlass]

Reasoning:

In most easy network configurations, one would have a set of initial seeds to facilitate peer discovery. In the current state of this library, those nodes would be overwhelmed with traffic. They would have nearly as many connections as there are nodes.

Mechanism:

When a node (A) gets a new connection, and already has more than 2 * max_outgoing connections, it should:

• Not send an initial handshake
• Do send a peerlist
• Send flags.disconnect

The flags.disconnect message tells the receiver (B) to initiate a disconnect. This would be remembered by A, and A would not react to its disconnection.

You could think of this as a slightly more complicated HTTP 303

Caveats:

This mechanism will need to be modified for the chord schema, as relative distances will begin to matter. In that schema, this will be handled by disconnect_least_efficient.

[LivInTheLookingGlass]

Not necessarily needed, but makes #130 much more feasible