Merge #12172: Bugfix: RPC: savemempool: Don't save until LoadMempool() is finished

Summary:
cb1e319 Bugfix: RPC: savemempool: Don't save until LoadMempool() is finished (Jorge Timón)

Pull request description:

Fixes https://github.com/bitcoin/bitcoin/issues/12142

The tests are a little bit slow, mempool_persist.py goes from about 20 s to about 120 s in my hardware.
Perhaps there's a better way to test this.

Tree-SHA512: 9e6c24b32a9cf3774e8f0bd81c035b0deb53fba5ac3eb2532d85900579d21cef8a1135b75a4fa0a9d883e3822eb35e7d4b47a0838abf99789039205041962629

Backport of Core PR 12172
https://github.com/bitcoin/bitcoin/pull/12172/files
Depends on D2946

Test Plan:
make check
test_runner.py mempool_persist

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2957