[chronik] Allow to bypass reading the spent_by info for txs outputs
Summary: The electrum endpoints don't use this info and this is a costly lookup, so we can save lots of time by skipping it.
Test Plan:
./test/functional/test_runner.py chronik_electrum_*
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D18282