[avalanche] Send getavaaddr messages to the manually added nodes

Summary: As per title, this makes it easier to test addresses propagation and make it possible to build private network. Only avalanche enabled peers are concerned.

Test Plan:

./test/functional/test_runner.py abc_p2p_getavaaddr

Reviewers: #bitcoin_abc, tyler-smith

Reviewed By: #bitcoin_abc, tyler-smith

Subscribers: tyler-smith

Differential Revision: https://reviews.bitcoinabc.org/D11322