rpl: RPL/trickle with only *one* thread

This PR is *based* on my previous work in PR #1839
and proposes an approach to reduce the thread count of RPL.

The current RPL/Trickle stack needs about 5 threads to handle tasks
like updating the trickle timer, routing entries and the transmission of
DAOs.

This PR modifies RPL to use only one thread with a looped `msg_recv()` call.
The message is then multiplexed to the right task.