MEDIUM: mux-fcgi: do not make an fstrm subscribe to itself on deferred shut

This is the port to FCGI of previous commit "MEDIUM: mux-h2: do not make
an h2s subscribe to itself on deferred shut".

The purpose is to avoid subscribing to the send_wait list when trying to
close, because we'll soon have to merge both recv and send lists. Basic
testing showed no difference (performance nor issues).
1 file changed