[chronik] Electrum: implement the mempool.get_fee_histogram() endpoint

Summary:
https://electrum-cash-protocol.readthedocs.io/en/latest/protocol-methods.html#mempool-get-fee-histogram

We simply use powers of 2 as a fee rate interval, which seems to be what fulcrum does. Note that the unit is sat/vB.

Test Plan:

./test/functional/test_runner.py chronik_electrum_blockchain

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18246