Do not free client by ObserverMediator.

Why have I done this? Wrong idea from the beginning.
Add test which set client to observer mediator.
Add test files for few callbacks by ObserverMediator.
Fix few issues in ItemIterator and ObserverClient interface.

 * return end iterator with Unknown flag;
 * pass begin iterator as copy in callback.
Fix tests after added new folder.
Add file with test constants.
Add counter how many times callbacks was called.
Add test on few handled callbacks.