Move SocketEvents logic to private method.

Summary:
This separates the select() logic from the socket handling logic, setting up
for a switch to poll().

Partial backport of Core PR14336
https://github.com/bitcoin/bitcoin/pull/14336/commits/28211a4bc9c65859b641b81a0541726a0e01988f

Depends on D6460

Test Plan:

ninja
ninja check-all
src/bitcoind -debug=net

Verify normal node behavior.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6461