Initial implementation of RadixTree

Summary: This implements a radix tree that can be read from and inserted into concurently. Deletion is not implemented at this stage.

Test Plan: Added unit tests for implemented features.

Reviewers: #bitcoin_abc, jasonbcox, Fabien

Reviewed By: #bitcoin_abc, jasonbcox, Fabien

Subscribers: Fabien, Mengerian, jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2311