new feature: clock sync protocol ftsp (WIP)

see #1538

<del>This PR will add an implementation of the Flooding Time Synchronization Protocol (FTSP) described in [1]. This PR depends on #1425 .</del>

[1] "The flooding time synchronization protocol", Maroti et.al. 2004

IMPORTANT: I will refactor this code in the next few weeks. I will ping this PR when the code is ready for a
code review. Don't waste your time by reviewing this code now.