Sort Config entries and use O(log N) lookup

Sort Config entries and use O(log N) lookup

Decrease running time for getStringList (and all other get methods) by
looking for configuration entries using binary search rather than
linear search through the configuration file.

Configuration lines are sorted by section, subsection, name in a
sorted list whenever the snapshot is rebuilt. Binary search is used to
locate an index in the middle of the values, then walk backwards to
find the first value in the range.

Given a configuration of file of 5000 distinct section/subsection/name
triplets (e.g. a Gerrit Code Review project.config configuration file
with 5000 unique access control rules), this new code is faster to
lookup each rule individually using getStringList():

  old setStringList() 194 usec avg
      getStringList() 196 usec avg

  new setStringList() 188 usec avg
      getStringList()  24 usec avg

Change-Id: Ic8907231868c18eb946b72f341a6b58666b70324