rpc: add return message to savemempool RPC
Summary:
Currently, if the user calls the savemempool RPC method, there is no way to know where the file was created (unless the user knows internal implementation details). This PR adds a return message stating the file name and path where the mempool was saved and changes mempool_persist.py to validate this new return message.
Backport of core#23398.
Test Plan:
./test/functional/test_runner.py mempool_persist
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D12599