This can be used to combine e.g. the platform's SecureRandom
implementation with our own, so that a weakness in either source doesn't
harm security as long as the other source is strong.

We don't need two separate executors for long-running IO threads.

Fixed a bug in SimplexMessagingIntegrationTest that should've caught
this.

This ensures that if two DatabaseExecutor tasks update the database and
broadcast events, the events are broadcast in the same order as the
updates occurred.

This may have been the cause of bug #69. Removed the wait-for-shutdown
code in DroidtoothPlugin that was added to attempt to avoid that bug.

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.