rpc: Remove mempool global from miner

Summary:

The miner needs read-only access to the mempool. Instead of using the
mutable global ::mempool, keep a immutable reference to a mempool that
is passed to the miner. Apart from the obvious benefits of removing a
global and making things immutable, this might also simplify testing
with multiple mempools.

Backport of core PR17781.

Depends on D7984.

This differs quite a bit from the original PR because we already passed
a mempool. This is mostly rename and factorization to make it looks more
like core.

Test Plan:

ninja all check-all
ninja bench-bitcoin

With clang:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7988