LoadMempool: Pass in load_path, stop using gArgs

Summary:
Also:

1. Have CChainState::LoadMempool and ::ThreadImport take in paths and pass it through untouched to LoadMempool.
2. Make LoadMempool exit early if the load_path is empty.
3. Adjust the call to ::ThreadImport in ::AppInitMain to correctly pass in an empty path if mempool persistence is disabled.

This is a partial backport of core#25487
https://github.com/bitcoin/bitcoin/pull/25487/commits/06b88ffb8ae7f2b2a93a32908cd80e77fafd270c

Depends on D15721

Test Plan: ninja all check-all bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15722