Turn mempool entry ref into a RCU pointer
Summary:
This makes it possible to build radix trees of these.
Depends on D15147.
Test Plan:
ninja all check-all
Run with ASAN.
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D15148