Track time between scan results & warn if a threshold is exceeded

[tjanson]

After Wifi ScanResults are received (& handled), there is a 2 second delay (see constant and code), then a new scan is triggered.

It is unclear how long the scan takes. This is presumably device-dependent. A quick glance at the logs shows intervals from 3 to 6 seconds.

However, we want to make sure that this interval is consistent, i.e., that there are no interruptions. Thus the time between scans (while logging is activated) should be tracked, and the user should be warned, if a certain threshold (e.g., 15 seconds) is exceeded.