[net] Allow disconnectnode RPC to be called with node id

Summary: backport of PR10143 from core + a few bugfixes.

Test Plan:

../qa/pull-tester/rpc-tests.py disconnect_ban.py

Reviewers: freetrader, CCulianu, #bitcoin_abc

Reviewed By: CCulianu, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D501