v1.6 - implement Thin Links, a lighter weight version of Fat Links; fully templatize ZoneProcessors

• 1.6
  • Remove TimeZone::kTypeBasicManaged and TimeZone::kTypeExtendedManaged
    and merge them into just regular TimeZone::kTypeBasic and
    TimeZone::kTypeExtended.
    • Significantly simplifies the implementation of TimeZone.
    • TimeZone no longer holds a reference to a ZoneProcessorCache, it
      holds only a reference to ZoneProcessor.
    • The binding of TimeZone to its BasicZoneProcessor or
      ExtendedZoneProcessor now happens early, inside the
      BasicZoneManager or the ExtendedZoneManager, instead of delaying
      it to various methods inside the TimeZone through the
      ZoneProcessorCache.
    • This change should be invisible to library clients.
  • Large internal refactoring of ZoneProcessor, no external change
    • Fully templatize BasicZoneProcessor into
      BasicZoneProcessorTemplate, and ExtendedZoneProcessor to
      ExtendedZoneProcessorTemplate.
    • Remove sentinel static ZoneEra anchor record which prevented
      easy templatization.
    • Remove direct dependency to const ZoneInfo*, replacing it with
      generic uintptr_t zoneKey.
    • Insert BrokerFactory indirection layer to provide mapping from a
      generic uintptr_t zoneKey to the corresponding ZoneInfoBroker.
    • Templatized classes now depend only on their respective
      Zone*Broker classes.
    • This change should be invisible to library clients.
  • Fix stale ZoneProcessor binding to TimeZone.
    • A dereferenced nullptr could crash the program if
      TimeZone::toTimeZoneData() was called immediately after calling the TimeZone::forZoneInfo()` factory method.
    • Some accessor methods in TimeZone (getZoneId(), printTo(),
      printShortTo()) could return incorrect values if the number of
      unique TimeZones used by an application is greater than the cache
      SIZE template parameter given to the ZoneManager.
      • The problem occurs because the ZoneProcessorCache will rebind
        a previously allocated ZoneProcessor to another TimeZone when
        it runs out of available processors in the cache.
  • Unlikely Breaking Change: Move ZoneRegistrar.h into internal/.
    • Rename BasicZoneRegistrar to basic::ZoneRegistrar.
    • Rename ExtendedZoneRegistrar to extended::ZoneRegistrar.
    • The class is an implementation detail which is used only by
      BasicZoneManager and ExtendedZoneManager. It was not exposed to
      the end user and should not cause any breaking changes.
  • Add support for Thin Links using optional linkRegistry[] parameter in
    the constructors of BasicZoneManager and ExtendedZoneManager.
    • The zonedb/zone_registry.h and zonedbx/zone_registry.h files
      now contain a kLinkRegistrySize and a LinkEntry kLinkRegistry[]
      array. Each record in the array contains a mapping of linkId to its
      zoneId.
    • The ZoneManager::createForZoneId() method will search the Thin Link
      registry if a zoneId is not found in the Zone registry.
    • See Zones and Links section in the
      USER_GUIDE.md.
  • Breaking Change: Rename ZoneManager::registrySize() to
    zoneRegistrySize().
    • Add ZoneManager::linkRegistrySize() method.
    • A ZoneManager can now hold 2 different registries: the Zone (and Fat
      Link) registry, and the Thin Link registry. So we need to
      distinguish between the 2 registries.
    • See the Default Registries section
      in the USER_GUIDE.md for an explanation of the Zone
      and Link registries.