[RCU] Implement RCUPtr, a smart pointer that free asynchronousely when other threads did synchronize.

Summary:
This will make the memory managaement for the RadixTree significantly easier and even allow the tree to share some of its innard with the outside world.

Depends on D2317

Test Plan: Adde unt tests for the various behaviors implemented.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2340