Attempt to decrease qdigest memory footprint

Hi,
Full discussion here https://groups.google.com/forum/#!topic/stream-lib-user/DuB-n4rU15Y.

I created benchmark for inserts: https://github.com/whiter4bbit/qdigest-bench. This benchmark does sequential inserts of 50000000 elements (`random.nextInt(120000)`).

    whiter4bbit@katrin stream-lib-fork % java -version
    java version "1.7.0_21"
    Java(TM) SE Runtime Environment (build 1.7.0_21-b11)
    Java HotSpot(TM) 64-Bit Server VM (build 23.21-b01, mixed mode)

Java options: `-server -Xmx1280m -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCDateStamps -Xloggc:qdigest.log`

I measured young collections count and young/old gen heap size at the end of each pass. Implementation, which uses Long2LongOpenHashMap don't have young gen collections at all (as well as old gen).

Results:

2.5.0-SNAPSHOT:

    whiter4bbit@katrin qdigest-benchmark % bin/bench.sh 10 | ruby bin/summarize.rb

    Insert
      Mean: 45238
      Stddev: 7632.287599402947
      Median: 42502.0

    Young gen GC count
      Mean: 0
      Stddev: 0.0
      Median: 0.0

    Young gen heap used (at the end)
      Mean: 1885
      Stddev: 0.0
      Median: 1885.0

    Old gen heap used (at the end)
      Mean: 0
      Stddev: 0.0
      Median: 0.0


2.4.0:

    whiter4bbit@katrin qdigest-benchmark % bin/bench.sh 10 | ruby bin/summarize.rb
    Insert
      Mean: 49892
      Stddev: 2827.6702424434147
      Median: 50452.0

    Young gen GC count
      Mean: 999
      Stddev: 61.66846844214635
      Median: 1021.0

    Young gen heap used (at the end)
      Mean: 39305
      Stddev: 24565.86045714662
      Median: 34994.5

    Old gen heap used (at the end)
      Mean: 486
      Stddev: 20.784609690826528
      Median: 488.0