[chronik] Rename estimate_fee to estimate_feerate_sats_per_kb

Summary: This is simply more accurate and will avoid errors going forward. 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/D18181