[chronik] Electrum: Implement the *.get_mempool endpoint

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

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/D18156