BUG/MEDIUM: mux-pt: Release the tasklet during an HTTP upgrade

BUG/MEDIUM: mux-pt: Release the tasklet during an HTTP upgrade

When a TCP connection is upgraded to HTTP, the passthrough multiplexer owning
the client connection is detroyed and replaced by an HTTP multiplexer. When it
happens, the connection context is changed (it is in fact the mux itself). Thus,
when the mux-pt is destroyed, the connection is not released. But, only the
connection must be kept. Everything else concerning the mux must be
released. Especially, the tasklet used for I/O subscriptions. In this part,
there was a bug and the tasklet was never released.

This patch should fix the issue #935. It must be backported as far as 2.0.