Fix includes in net.h related to past avalanche refactors

Summary:
Delegation used to be used in the net code but was refactored out.
The delegation header just happens to provide the necessary headers indirectly.
This patch replaces it with the correct ones.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11553