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.

Very important stuff.

Bug #47.

Of course, not all devices support Bluetooth...

This allows the oldest LocalAuthor to be used as the default.

All JVMs must support UTF-8 encoding.

This adds complexity but will save a lot of bandwidth, as most of the
strings and byte arrays we want to send are less than 128 bytes.

The extra complexity isn't exposed outside of the serial component.

We check for compatibility by specifying a schema version and minimum
schema version in the code, storing them in the DB, and checking whether
the DB is too old for the code or vice versa.

Dev task #52. Known but unverified identities are also supported, but
currently unused. These will be used in future for contacts who've been
introduced but not verified face to face.

This should've been changed when restricted groups were removed.