Add IPv4 preference support

[martinreichart]

In our tests with Austrian time servers we often see that using IPv6 addresses results in much slower query times (consistently 10 seconds instead of ~4 with only IPv4).

This PR adds a new parameter to the Clock.sync call and NTPClient to allow the preference of IPv4 addresses if both IPv4 and IPv6 addresses are resolved.

[Reflejo]

Sorry for the delay, not a big fan of this. The way this is usually implemented (happy eye balls, etc) is to prioritize one over the other but not shut down one category. One solution could be to sort the dns results by preferred category and pick the result of the first N successful ones, regardless if they are ipv4 or 6.

could have a [.preferIPv6, .preferIPv4, .auto] sort of setting