[RCU] Make it possible to serialize/deserialize a RCUPtr

Summary: Similar to what is done for other smart pointers, this will serialize the contained object and build a RCUPtr from the deserialized object.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11514