rpc: Expose g_is_mempool_loaded via getmempoolinfo

Summary:
And use it to fix a race condition in mempool_persist.py:
https://travis-ci.org/Empact/bitcoin/jobs/487577243

Backport of Bitcoin Core PR15323
https://github.com/bitcoin/bitcoin/pull/15323

Depends on D4234 and D4266.

Test Plan:

ninja check-all

• Read the docs.

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: Fabien, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4243