guggero / btcd / 27423950121

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netsync: require block-serving services on regtest/simnet sync peers

Commit 26124d275 made every peer a sync candidate on regtest and
simnet so that nodes on non-localhost networks (e.g. Docker bridge
networks) can be synced from. Dropping the address requirement was
the intent, but the change also dropped the service-flag requirement,
so light clients became eligible sync peers.

A light client (e.g. neutrino) advertises a recent best height but
can serve neither headers nor blocks. Electing one as the sync peer
stalls the sync until the stall handler disconnects it, and with
other light client connections present the next one is elected and
stalls again, livelocking the sync indefinitely. This surfaced in
neutrino's sync tests, where a btcd simnet node connected to both a
neutrino instance and other btcd nodes never synced.

Keep accepting any peer address on regtest/simnet, but require the
peer to signal SFNodeNetwork or SFNodeNetworkLimited like on any
other network.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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