Adds PeerMessagingState enum to seeder/bitcoin.*

Summary:
Moves enum from just the p2p_messaging_tests.cpp to bitcoin.h allowing
use in the rest of the seeder code.

Test Plan:

ninja check-bitcoin-seeder

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5458