Move GenerateSelectSet logic to private method.

Summary:
This separates the socket event collection logic from the logic
deciding which events we're interested in at all.

Partial backport of Core PR14336
https://github.com/bitcoin/bitcoin/pull/14336/commits/7e403c0ae705455aa66f7df9a9a99f462fd4e9a8

Depends on D6459

Test Plan:

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

Verify normal node connection behavior

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

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

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6460