- replaced use of Object instance mutex with a private final Lock object
- replaced Object signaling with specific condition signalling

See bug #27.

The TransportReader/Writer's dispose() method should handle that, and
ConnectionManager is responsible for calling it.

This makes it more likely that connections will be closed cleanly.
However, the interrupt() method is currently ineffective for incoming
sessions as it won't interrupt a blocking read, e.g. when the packet
reader is waiting for a packet.

Because that's what it does.

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.