[RCU] Add an implicit converting constructor

Summary: This will create an RCUPtr<T> from an RCUPtr<U> if U * is implicitely convertible to T *. This will come in handy for converting RCUPtr<T> to RCUPtr<const T>.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11509