Improve MergeIterator performance

The implementation of MergeIterator uses a priority queue and applies a pair of poll+add operations for every item in the resulting sequence. This is quite inefficient as poll necessarily applies at least log N comparisons (up to 2log N), and add often requires another log N, for example in the case where the inputs largely don't overlap (where N is the number of iterators being merged).

This can easily be replaced with a simple custom structure that can perform replacement of the top of the queue in a single step, which will very often complete after a couple of comparisons and in the worst case scenarios will match the complexity of the current implementation.

This should significantly improve merge performance for iterators with limited overlap (e.g. levelled compaction).