Master internals: Arrange threads in a single array.

- Add RouterThread::unchecked_thread().

- Add Master::root_router(). This returns a background router that exists
  as long as the Master itself does (making it useful for Tasks, Timers,
  etc. that aren't associated with a specific router).

Signed-off-by: Eddie Kohler <ekohler@gmail.com>
Initial framework for read-copy update.

Signed-off-by: Eddie Kohler <ekohler@gmail.com>
Task: Improve documentation for fast_reschedule().