[Radix] Use RCUPtr to keep track of items in the tree

Summary:
This ensures that ownership can be shared in between the tree and its users, and ensures that elements gets cleaned up properly as all owners get rid of references to them.

The tree is steal leaking it's internal nodes on destructions.

Depends on D2336, D2351 and D2352

Test Plan: Added test cases.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2371