[RCU] Add new features to RCUPtr

Summary:

• Make constructor private and add factory methods to clear up ownership.
• Add a release method for the RCUPtr to give up ownership.
• Add get method to get the underlying pointer.
• Add operator* overloads.
• Add an explicit convertion to bool that checks for null

Test Plan: Added tests cases that uses all these methods.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2350