new feature: gtimer; a global timer/clock for clock sync protocols (WIP)

see #1538

<del>This PR will add gtimer an adjustable clock which can be used in conjunction with clock sync protocols
in order to provide a network-wide synchronized timer.
Currently it is just an adjustable clock (rate and value can be changed).
However, we plan to add some vtimer like capabilities to it later on.</del>

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.