[net/refactor] Add connection type as a member var to CNode

Summary:

- Directly maintaining the connection type prevents having to deduce it
from several flags.

Partial backport (9/19) of core PR19316:
https://github.com/bitcoin/bitcoin/pull/19316/commits/d3698b5ee309cf0f0cdfb286d6b30a256d7deae5

Depends on D8718.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8719