extensions fix deflate stream vs mux and veto it on mux children

extensions fix deflate stream vs mux and veto it on mux children

This patch gets deflate-stream working with x-google-mux.

It adds a clean veto system where are extension can veto the proposal
of any extension when opening a new connection.  x-google-mux uses that
in its callback to defeat any use of deflate-stream on mux children.

However deflate stream is allowed on the parent connection and works
transparently now alongside x-google-mux.

Signed-off-by: Andy Green <andy@warmcat.com>