v0.15.0

This is a 1.0 release candidate - please try it out and give us any last-minute feedback, especially on new and changed APIs.

• New Box putAsync() returning a Future and putQueued() for asynchronous writes.
• Query now supports auto-closing. You can still call close() manually if you want to free native resources sooner
  than they would be by Dart's garbage collector, but it's not mandatory anymore.
• Change the "meta-model" fields to provide completely type-safe query building.
  Conditions you specify are now checked at compile time to match the queried entity.
• Make property queries fully typed, PropertyQuery.find() now returns the appropriate List<...> type without casts.
• Query conditions inside() renamed to oneOf(), notIn() and notInList() renamed to notOneOf().
• Query stream and findStream() are replaced by QueryBuilder.watch(), i.e. box.query(...).watch().
• New Query stream() to stream objects all the while the query is executed in the background.
• Store subscribe<EntityType>() renamed to watch().
• Store subscribeAll() replaced by a shared broadcast stream entityChanges.
• Entities can now contain final fields and they're properly stored/loaded (must be constructor params).
• Flutter desktop - native library is now downloaded automatically, same as for mobile platforms.
• Follow exception-vs-error throwing conventions - throwing errors when it's a permanent developer-caused error. Namely,
  there's a new UniqueViolationException thrown when an object you're trying to put() would violate a Unique() index.
• Even higher than usual amount of internal optimizations and improvements.
• Update to objectbox-c v0.14.0.
• Update to objectbox-swift v1.6.0.
• Update to objectbox-android v2.9.2-RC.