introduced a generosity factor that makes the search less greedy.
it dramatically improves the recall by making the search a bit slower
(for the same recall one can use half the M and smaller ef).
had to add Queue::safe_push() method that removes one of the
furthest elements (not necessarily the furthest) in the queue
to keep it from overflowing.
Bounded_queue<> pretended to be a typesafe C++ wrapper
on top of pure C queues.h.
But it wasn't, it was tightly bounded to filesort and only useful there.
* implement Queue<> - a typesafe C++ wrapper on top of QUEUE
* move Bounded_queue to filesort.cc, remove pointless "generalizations"
change it to use Queue.
* remove bounded_queue.h
* change subselect_rowid_merge_engine to use Queue, not QUEUE
