Refactor PerfCounterQuery classes and fix memory leak

[dbwiddis]

Fixes #1244

[coveralls]

Coverage remained the same at 88.398% when pulling 193bd6c on dbwiddis:memleak into 92ca9b8 on oshi:master.

[codecov]

Codecov Report

Merging #1245 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1245   +/-   ##
=========================================
  Coverage     85.85%   85.85%           
  Complexity      279      279           
=========================================
  Files            25       25           
  Lines           905      905           
  Branches        124      124           
=========================================
  Hits            777      777           
  Misses           73       73           
  Partials         55       55           

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