the TopNCollector now can either use a heap or a list
i did not code it to use an interface, because this is a very hot
loop during searching. rather, it lets bleve developers easily
toggle between the two (or other ideas) by changing 2 lines
The list is faster in the benchmark, but causes more allocations.
The list is once again the default (for now).
To switch to the heap implementation, change:
store *collectStoreList
to
store *collectStoreHeap
and
newStoreList(...
to
newStoreHeap(...
