implement distributed clock synchronized to master

[jimy-byerley]

#20 implemented the DC synchronizing the slaves clocks to the reference slave, which is the most precise DC mode. However an other way of doing is possible: synchronizing the slaves clocks to the master's clock (it actually suffers from the ethernet delays between the master and the first slave). but avoids correcting the master clock to follow the reference.

This feature could be useful to some usecases.
Implementing this should not need any addition activation on the salves, just changing the way time is distributed to slaves in DistributedClock