0.6 - simplify Clock classes

• 0.6
  • Update tests to use UnixHostDuino.
  • Fix broken restore functionality in CommandLineClock. Make it work
    on Unix using UnixHostDuino. Make it work on ESP8266 and ESP32 again.
  • Update flash memory consumption numbers in zonedb/zone_infos.cpp and
    zonedbx/zone_infos.cpp.
  • Lift ace_time::common::DateStrings to just ace_time::DateStrings
    because it was the only data/time class in the common:: namespace so
    it seemed inconsistent and out of place.
  • Breaking Change Large refactoring and simplification of the
    ace_time::clock classes. Merged TimeKeeper and TimeProvider into a
    single Clock class hierarcy. Merged SystemClockSyncLoop (separate
    class) into SystemClockLoop (subclass of SystemClock) and
    SystemClockSyncCoroutine (separate class) into SystemClockCoroutine
    (subclass of SystemClock with mixin of ace_routine::Coroutine). Merged
    keepAlive() into ::loop() and ::runCoroutine() methods, so we don't
    need to worry about it separately anymore. Made SystemClockLoop use the
    non-blocking methods of Clock, making it as responsive as
    SystemClockCoroutine.
  • Add UnixClock.h which provides access to the internal Unix clock
    when using UnixHostDuino.