Signed-off-by: goapunk <noobie@goapunks.net>

Briar crashed when run in a device without bluetooth or without camera
such as an emulator.

Closes #514

Modified the project structure, removed module extension and went instead for a non-complete core dependency graph

When more than one invitation socket is opened, Alice should pick which one to use and Bob should use whichever one Alice picks. This fixes a race condition where each party picked a different socket and closed the other.

The polling interval increases exponentially each time polling is unsuccessful, up to a maximum of 60 minutes. The interval is reset to 2 minutes whenever a connection is made and whenever Bluetooth is re-enabled.

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.

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

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 may have been the cause of Bluetooth not always being disabled at
shutdown on the Sony Xperia Tipo.