Improvements for `BlackBoxBoundedContext`

[alexander-yevsyukov]

• Do not descend BlackBoxBoundedContext from AbstractEventSubscriber. This adds unnecessary public API to the class, which is confusing for the user. Have a package-private class which does all the job instead.

• Deprecate all the factory methods, leaving only from()

• Remove deprecated API.

• Why do we want to expose SingleTenantBlackBoxContext and MultiTenantBlackBoxContext as separate types? Can we keep the package-private? It would improve debugging, but having isMultitenant() is quite enough for the public API. After all we want to make the tenancy aspect hidden from the users as much as possible. Let's be consistent.

• Remove generic parameter of BlackBoxContext.

• Remove test-only methods like eventBus(), commandBus(), stateTypes().

• Remove withActorIn(UserId userId, ZoneId zoneId, ZoneOffset zoneOffset) method.
  Use combination withActor() and in() methods instead.

• Remove zoneOffset parameter from in() method.
  Obtain zoneOffset from the passed zoneId.

[alexander-yevsyukov]

Partially addressed in #1251.

[alexander-yevsyukov]

Finalized in #1252.