Use best-fit strategy in Arena, now O(log(n)) instead O(n)

This replaces the first-fit algorithm used in the Arena with a best-fit. According to "Dynamic Storage Allocation: A Survey and Critical Review", Wilson et. al. 1995, http://www.scs.stanford.edu/14wi-cs140/sched/readings/wilson.pdf, both startegies work well in practice.

The advantage of using best-fit is that we can switch the slow O(n) algorithm to O(log(n)) operations. Additionally, some previously O(log(n)) operations are now replaced with O(1) operations by using a hash map. The end effect is that the benchmark runs about 2.5 times faster on my machine:

old: BenchLockedPool, 5, 530, 5.25749, 0.00196938, 0.00199755, 0.00198172
new: BenchLockedPool, 5, 1300, 5.11313, 0.000781493, 0.000793314, 0.00078606

I've run all unit tests and benchmarks.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/bench/lockedpool.cpp b/src/bench/lockedpool.cpp
index 914e37a2ed..8b7fe5f609 100644
--- a/src/bench/lockedpool.cpp
+++ b/src/bench/lockedpool.cpp
@@ -43,4 +43,4 @@ static void BenchLockedPool(benchmark::State& state)
     addr.clear();
 }
 
-BENCHMARK(BenchLockedPool, 530);
+BENCHMARK(BenchLockedPool, 1300);