Implement COW for the radix tree

Summary:
As per title.

It is now possible to "copy" a radix tree, but noting is actually copied. Instead only the node that get writtent o end up being lazily duplicated.

Test Plan: Added unit tests to cover the COW and move semantic.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11392