Improve the performance of Application.sortRequests()

Too much time can be spent in Application.sortRequests(). There are various ways to solve this problem.

Two competing approaches:

1. Store requests in a sorted data structures (btree.BTree from Google)
2. Only sort if a new request is added. If pendingAskRepeat becomes 0, simply skip it.

Based on benchmarks, #2 is much simpler and even faster than the BTree based solution, even for 100,000 requests (that is, we go through the whole slice and only process asks where pendingAskRepeat > 0).

BenchmarkBtree_SmallItemCount
BenchmarkBtree_SmallItemCount-6                    	 2143888	       553.1 ns/op	      80 B/op	       4 allocs/op
BenchmarkIteration_DontRemovePendingAskRepeat0
BenchmarkIteration_DontRemovePendingAskRepeat0-6   	 5071994	       253.9 ns/op	       0 B/op	       0 allocs/op
BenchmarkIteration_AlwaysSort
BenchmarkIteration_AlwaysSort-6                    	    3002	    395433 ns/op	     304 B/op	       7 allocs/op

BenchmarkBtree_SmallItemCount walks a btree with 10 elements.
BenchmarkIteration_DontRemovePendingAskRepeat0 goes through a slice of 100,000 elements but only 10 of them are interesting to us.
BenchmarkIteration_AlwaysSort is what we have now.

If we factor in adding requests / re-sorts, it's still better to sort.