WL#2571: replace a tree with priority queue in Natural Language Full-Text Search

Try to use priority queue in NL FTS, just as it's done in the boolean mode.

Benefits:
 * no memory limits for large result sets
 * no O(log N) tree speed penalty
 * rows are found one by one - no expensive init step
 * LIMIT works better

Drawbacks:
 * rows are not automatically sorted by relevance