Improve ping script output readability

[ThirteenFish]

Previously each ping iteration could print up to three lines, and it was sometimes easy to miss if the latest iteration succeeded or failed. This tries to limit each iteration to only a single vertically aligned line, printing information as it comes in.

While I was here I simplified the implementation:

• By being a little more careful about where and how long blocking happens I was able to drop multiple threads. This makes coordinating printing lines and shared data much easier
• Removed previous loop timestamp tracking, it didn't seem to work anyway?

(unit tests are failing for unrelated reasons, because of the updated cfdp-py dependency. Fixed when #44 is merged.)

[ryanpdx]

This is simpler, but it does remove some functionality, tho the need for that functionality is an edge case.

The new style seems to expect one or none ping responses between sent pings while the old style could handle receiving any response at any time.