Introduce configurable percentiles for wavefront sink

[blt]

This commit makes it possible for the end-user to specify what
percentiles will be shipped out in the wavefront sink. The config
for doing so is... goofy. This problem is discussed in #221.

This commit resolves #218.

Signed-off-by: Brian L. Troutwine blt@postmates.com

[blt]

/cc @bitglue

[codecov]

Codecov Report

Merging #222 into master will increase coverage by 0.48%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
+ Coverage   96.55%   97.03%   +0.48%     
==========================================
  Files          22       22              
  Lines        2841     2967     +126     
==========================================
+ Hits         2743     2879     +136     
+ Misses         98       88      -10

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[gliush]

LGTM, though, I don't like that we have the same percentiles initialisation code (though, I realise that right now we can't fix it).