collection priority queue: head should be higher priority than tail

[vagabondkjo]

This PR makes some changes as below

1. collection priority queue

• head should be higher priority than tail

2. add sint64 headIndex(const id& pov, sint64 maxPriority)

• return elementIndex of first element with priority <= maxPriority in priority queue of pov (or NULL_INDEX if pov is unknown).

3. add sint64 tailIndex(const id& pov, sint64 minPriority)

• return elementIndex of last element with priority >= minPriority in priority queue of pov (or NULL_INDEX if pov is unknown).

[philippwerner]

Thanks! Looks good at the first glimpse, but please add some more tests to improve the test coverage and make sure that the code works correctly on more cases. These breakpoints are never reached in the tests:

[vagabondkjo]

Thanks! Looks good at the first glimpse, but please add some more tests to improve the test coverage and make sure that the code works correctly on more cases. These breakpoints are never reached in the tests:

@philippwerner thanks for your comment.

I make some changes for _headIndex/_tailIndex, I think it's better than previous implementation. And, I also add more unittests.
Please help to review the PR again. Thanks!