apply host backoff to non-http protocols

#7793 wires the per-host backoff governor (pkg/protocols/common/hostbackoff) into http only, since 429, 503 and repeated 403s are http signals.

The governor is protocol-agnostic. Other protocols currently feed transport failures to hosterrorscache, which skips a host rather than slowing it, so the same connection-failure signal could drive a backoff instead.

Follow-up to #7783.