add QTreeSerializer

[nevillelyh]

Default kryo serialization for QTree is not deterministic due to other fields in the class, and this causes problem in some frameworks like Apache Beam which detects mutation of input element via serialization.

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #305 into develop will increase coverage by 0.46%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #305      +/-   ##
===========================================
+ Coverage    52.13%   52.59%   +0.46%     
===========================================
  Files           42       42              
  Lines         1314     1327      +13     
  Branches        38       38              
===========================================
+ Hits           685      698      +13     
  Misses         629      629

Impacted Files	Coverage Δ
...com/twitter/chill/algebird/AlgebirdRegistrar.scala	100% <100%> (ø)	⬆️
...m/twitter/chill/algebird/AlgebirdSerializers.scala	100% <100%> (ø)	⬆️

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[johnynek]

Thanks!