Merge #12415: Interrupt loading thread after shutdown request

Summary:
2e9406c Interrupt loading thread after shutdown request (João Barbosa)

Pull request description:

This change (currently) avoids loading the mempool if shutdown is requested.

Tree-SHA512: 3dca3a6ea5b09bd71db0974584d93dfe81819bc0bdbb4d9b6fa0474755306d1403f6c058ecb8211384493a8f7ca3a9134173db744b7344043cfc7d79286c8fd4

Backport of Core PR12415
https://github.com/bitcoin/bitcoin/pull/12415/files

Test Plan:
make check
test_runner.py
bitcoind --testnet -help-debug -stopafterblockimport # shuts down as expected with this message in the logs: Stopping after block import

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2958