Add ability to ping more frequently

[AlfieJ]

We'd like to have more detailed information about when and for how long a device goes offline.

To do this, ping more frequently, but instead of storing the full set of ping results store just the differences since the last ping.

• I suspect the best thing is to change our pinging so that it happens in the background (in a separate thread?) and automatically pings everything once a minute, instead of going off the pinging interval that is specified in the config tool.
  • We'd continue to use the pinging interval, however, to specify how often the full ping results should be stored.
  • Maybe we just do the pinging once a minute since the actual pinging does take place within separate threads, then 'collect' the results in the main thread so that we're not having to deal with the data coming from a different thread.