fix setCollectionViewLayout method's slowly & contentOffset setting

PSTCollectionView's setCollectionLayout method set contentOffset to top of content, 
But UICollectionView's setCollectionLayout method set contentOffset to point of center of visible item.
So I fixed setCollectionLayout's contentOffset setting.

And the setCollectionLayout has slowly problem with more items. (https://github.com/steipete/PSTCollectionView/issues/161)
So I fixed the problem by getting newly visible layout attributes in appropriate bounds.

And fixed _allVisibleViewDict removing bugs.