[electrum] add more typehints for blockchain related functions

Summary:
And refactor can_connect -> get_connecting_chain because it return a Blockchain that is used in network.py (same rationale as get_chain_for_header refactoring in D14950)

Depends on D14951

Test Plan:

python test_runner.py
pytest electrumabc/tests/regtest

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14952