BUG/MEDIUM: mux-h2: don't refrain from offering oneself a used buffer

BUG/MEDIUM: mux-h2: don't refrain from offering oneself a used buffer

Usually when calling offer_buffer(), we don't expect to offer it to
ourselves. But with h2 we have the same buffer_wait for the two directions
so we can unblock the recv path when completing a send(), or we can unblock
part of the mux buffer after sending the first few buffers that we managed
to collect. Thus it is important to always accept to wake up any requester.

A few parts of this patch could possibly be backported but earlier versions
already have other issues related to low-buffer condition so it's not sure
it's worth taking the risk to make things worse.