Merge #12842: Prevent concurrent savemempool

Summary:
585b47cfe133fae112782ad0a88fe25c71d465fa rpc: Prevent concurrent savemempool (João Barbosa)

Pull request description:

Follow up of #12172, this change prevents calling `savemempool` RPC concurrently.

Tree-SHA512: 4759a7107658a9794f5c6ab7e3e3002276abadd64996010be67a2791914d284db6fe0377c071a8d6a42387bfb0178f219b73aeec077ce5c4fe5c634a30b3e081

Backport of Core PR12842
https://github.com/bitcoin/bitcoin/pull/12842/files
Depends on D2946, D2996

Test Plan:
make check
test_runner.py

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2995