Move SocketHandler logic to private method.

Summary:
This is a partical backport of Core PR14335 : https://github.com/bitcoin/bitcoin/pull/14335/commits/032488e6e7b4c2d37f3f434d575d25806ddcb0b0

Depends on D4088

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4089