Fix mutable priority queue being wiped when moving out of function
Without move constructor, the clean() gets called when returning an instance from a function.
The above fix was applied also to MutableSkipHeapPriorityQueue.
Follow-up to 15a082b80b1d9ed3cfa70d3d8133c4cd1e5d699d:
Fixed TEST_CASE("Mutable priority queue - first pop", "[MutableSkipHeapPriorityQueue]")
Improved readability by introducing invalid_id() getter.
Made the ResetIndexWhenRemoved flag active in both debug and release mode,
it used to be made active by Vojtech for release mode only for unknown
reason.
