[chronik] Electrum: send the tip header upon block disconnection

Summary: This better replicates the behavior of fulcrum, and Electrum expects it during reorgs.

Test Plan:

./test/functional/test_runner.py chronik_electrum*

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18286