compaction_policy: avoid a division in approximation algorithm
compaction_policy: avoid a division in approximation algorithm

This does some simple mathematical substitution to avoid an unnecessary
division instruction in the approximation algorithm in the compaction
policy. This provided a ~5% speedup:

Before:

 Performance counter stats for 'build/latest/bin/compaction_policy-test --gtest_filter=*TinyOver*' (10 runs):

        426.861117      task-clock (msec)         #    0.999 CPUs utilized            ( +-  0.87% )
                 2      context-switches          #    0.004 K/sec                    ( +- 23.13% )
                 0      cpu-migrations            #    0.000 K/sec
             2,476      page-faults               #    0.006 M/sec                    ( +-  4.38% )
     1,597,139,052      cycles                    #    3.742 GHz                      ( +-  0.67% )
     4,013,990,056      instructions              #    2.51  insn per cycle           ( +-  0.01% )
       628,035,462      branches                  # 1471.288 M/sec                    ( +-  0.01% )
         2,408,063      branch-misses             #    0.38% of all branches          ( +-  0.13% )

       0.427287921 seconds time elapsed                                          ( +-  0.86% )

After:

 Performance counter stats for 'build/latest/bin/compaction_policy-test --gtest_filter=*TinyOver*' (10 runs):

        404.664720      task-clock (msec)         #    0.999 CPUs utilized            ( +-  0.93% )
                 1      context-switches          #    0.003 K/sec                    ( +- 28.21% )
                 0      cpu-migrations            #    0.000 K/sec
             2,531      page-faults               #    0.006 M/sec                    ( +-  3.54% )
     1,508,535,824      cycles                    #    3.728 GHz                      ( +-  0.22% )
     3,916,607,007      instructions              #    2.60  insn per cycle           ( +-  0.01% )
       628,048,996      branches                  # 1552.023 M/sec                    ( +-  0.01% )
         2,380,872      branch-misses             #    0.38% of all branches          ( +-  0.08% )

       0.404947434 seconds time elapsed                                          ( +-  0.94% )

Change-Id: I018a81987744c35ff672765ac9e212fd3654638d
Reviewed-on: http://gerrit.cloudera.org:8080/8445
Tested-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

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