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 PR 12842
https://github.com/bitcoin/bitcoin/pull/12842/files
Depends on D2946, D2996