[chronik] Electrum: faster status computation

Summary:
This avoid building a string with expensive copies before hashing the data. Also avoid the use of a copy iterator which is not needed.
There is no change in behavior.

Test Plan:

./test/functional/test_runner.py chronik_electrum_*

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18280