context: hashtrie based keyvalue store (#3368)

This is the hashtrie data structure authored by @ejona86

The linked list key value store is known cause problems in
pathological cases where users keep updating the same key(s) over and
over. This copy on write tree will bound reads at O(lgN) where N is
the number of keys in the map, rather than O(lgM) where M is the total
number of put operations.

Also:

- added some unit tests
- ran a test putting random keys into the map and comparing the result
  with a java.util.HashMap to verify sanity. The test passes but I
  won't check it into the repo because it takes a long time to run:
  https://gist.github.com/zpencer/12cb435235d171c1fe09aef18825fad0