rpc: Return total fee in mempool

Summary:
Also, add missing lock annotations

This is a backport of core#20944

Depends on D12067

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12058