Constant number of probes when using -Q reporting

[schweikert]

As a followup to the discussion in #105:

When a -Q interval is reported, it happens that for some intervals there is a certain number of probes, and for other intervals another. It would be nice if the reported number of sent pings for each interval was the same.

Idea on how to achieve this:

1. Don't send further pings if too near to the reporting-time
2. Try to sync the send-time and the reporting-time. This should be possible by noticing that the next ping is near to the reporting time, and slightly adjusting the time.

[schweikert]

fixed in #193