[Chronik] Add test for disconnectpool

Summary: Add a test to ensure txs and blocks are added to Chronik in the right order when doing a reorg with mempool txs present. If e.g. a mempool tx would be added before blocks, Chronik would crash under stome circumstances, which is currently not tested.

Test Plan: ./test/functional/test_runner.py chronik_mempool_disconnectpool

Reviewers: Fabien, #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16534