p2p: add MAX_FEELER_CONNECTIONS
Summary:
p2p: add MAX_FEELER_CONNECTIONS constan
test: use p2p constants in denial of service tests
This is a backport of Core PR18910
Test Plan: ninja && ninja check
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D9096