BUG/MINOR: connection/debug: do not enforce !event_type on subscribe() anymore

When building with DEBUG_STRICT, there are still some BUG_ON(events&event_type)
in the subscribe() code which are not welcome anymore since we explicitly
permit to wake the caller up on readiness. This causes some regtests to fail
since 2c1f37d353 ("OPTIM: mux-h1: subscribe rather than waking up at a few
other places") when built with this option.

No backport is needed, this is 2.2-dev.
5 files changed