[RCU] Make RCUPtr compatible with const payloads.

Summary:
As per title.

Depends on D2350

Test Plan: Added tests cases.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2352