Make sure to properly clamp delay times, as the recent issue had shown
it is possible to go below the update limit. SNTP app does not do any
runtime checks, so it's up to us to enforce these limits
Update internal random number generator from raw `secureRandom()` to a custom
'std::random_device'-look-alike, where we could also use it with STL functions
(current implementation is `os_random()` aka `esp_random()`)