Move DisconnectNodes logic to private method.

Summary:
This is a partical backport of Core PR14335 : https://github.com/bitcoin/bitcoin/pull/14335/commits/7479b63d9109ccaf3708a92dc65dc24556576551

Depends on D4086

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4087