Outbound connection-type ladder stalls while stale peers hold the full-relay target #399

A demoted stale outbound peer keeps its connection type but is excluded from nOutboundFullRelay, so while stale peers hold the outbound-full-relay target that count stays below m_max_outbound_full_relay. ThreadOpenConnections therefore takes the full-relay arm of the connection-type ladder on every pass, and block-relay, feeler and extra-network connections are never reached.

Such a node has no block-relay-only peers, writes no anchors, and never probes addrman with feelers. On regtest, holding 8 stale full-relay peers with no NODE_BLAKE2B peer reachable, after 5 minutes:

connection types: {'outbound-full-relay': 8}
feeler connections made: 0

It is reachable whenever the node cannot fill its full-relay target with fork-capable peers. On mainnet over 8 minutes a 29.x-knots node held no block-relay-only peers at all, while with #398 it held 3 by 181s, filling the target being what lets the ladder advance. A node that can find no fork-capable peer stays at nOutboundFullRelay == 0.