Revision 711559

* Fix issue Felix-803:
  The core xml schema accepted the 'interface' attribute in service dependencies. This attribute were replaced by 'specification' to be consistent with other service dependencies.
  The 'interface' attribute is now prohibited.

* Fix issue Felix-805:
  Instances were not created when the targeted factory becomes valid later.

* Add a mechanism to avoid that setter methods are called twice if two threads try to invoke the callback with the same value.