bp_worker: use SpawnServerClient::ReplaceSocket()
bp_worker: use SpawnServerClient::ReplaceSocket()

Fixes a crash bug that occurs when using worker processes.  The
problem is that we can't change BpInstace::spawn_service, because this
pointer has been propagated to a lot of subsystems.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy