BUG/MEDIUM: proxy: Don't use cs_destroy() when freeing the conn_stream.

BUG/MEDIUM: proxy: Don't use cs_destroy() when freeing the conn_stream.

When we upgrade the mux from TCP to H2/HTX, don't use cs_destroy() to free
the conn_stream, use cs_free() instead. Using cs_destroy() would call the
mux detach method, and at that point of time the mux would be the H2 mux,
which knows nothing about that conn_stream, so bad things would happen.
This should eventually make upgrade from TCP to H2/HTX work, and fix
the github issue #196.

This should be backported to 2.0.