bp_instance: add cleanup code to destructor
bp_instance: add cleanup code to destructor

Fixes assertion failures from the memory leak detector when an
exception gets thrown before the event loop is reached (and the
shutdown callback is not invoked, which usually does this cleanup).

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