MINOR: muxes: Pass the context of the mux to destroy() instead of the connection

MINOR: muxes: Pass the context of the mux to destroy() instead of the connection

It is mandatory to handle mux upgrades, because during a mux upgrade, the
connection will be reassigned to another multiplexer. So when the old one is
destroyed, it does not own the connection anymore. Or in other words, conn->ctx
does not point to the old mux's context when its destroy() callback is
called. So we now rely on the multiplexer context do destroy it instead of the
connection.

In addition, h1_release() and h2_release() have also been updated in the same
way.