sch: inconsistent DST <-> SDT transition when searching

- probe for double and missing hours when daylight saving time happens
  this is specific to local time schedules, utc should remain unaffected
- try to stay consistent with current pattern matching happening in loop
  when an hour is missing, simply skip to the next schedule
  when hour is repeated, make sure to recheck 'earlier' matches
- clean-up tests previously using very-very internal structs
  try to handle everything (sic) using `handle_*` functions
  from the corresponding namespace