Same exponential backoff approach as Bluetooth.

DuplexOutgoingSession flushes its output stream if it's idle for a
transport-defined interval, causing an empty frame to be sent. The TCP
and Tor plugins use a socket timeout equal to twice the idle interval to
detect dead connections.

See bugs #27, #46 and #60.

This should enable us to connect to contacts faster at startup and
whenever a new means of connecting becomes available.

The LAN plugin only accepts IPv4 link-local or site-local addresses.
This rules out LANs that use globally routable addresses (such as UCL),
but also reduces the chances of the LAN plugin making observable
connections across the WAN, which could reveal the social graph.

Both plugins will attempt to reuse the previous address and port only if
there's currently an interface with that address; this will avoid
unnecessary attempts to bind to nonexistent addresses.

Of course, not all devices support Bluetooth...