Merge #11099: [RPC][mempool]: Add savemempool RPC

Summary:
1aa97ee08 Add savemempool RPC (Lawrence Nahum)
467cbbcbf Add return value to DumpMempool (Lawrence Nahum)

Pull request description:

Adds a simple parameterless rpc command to dump the mempool.

Rationale:

Sometimes there can be a crash for whatever reason (bug, power loss, etc) causing the mempool.dat file to not be saved.

This change allows to script/cron the rpc call to have more regular saves to the file as well as cli/ad-hoc.

This should solve issue https://github.com/bitcoin/bitcoin/issues/11086

Tree-SHA512: e856ae9777425a4521279c9b58e69285d8e374790bebefd3284cf91931eac0e456f86224f427a087a01bf70440bf6e439fa02c8a34940eb1046ae473e98b6aaa

Backport of Core PR 11099
https://github.com/bitcoin/bitcoin/pull/11099/files

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/D2946