MINOR: connection: implement conn_release()

MINOR: connection: implement conn_release()

Several places reuse the same code to ensure a connection is properly
freed, either via its MUX or by calling the proper set of functions.
Factorize all of this in a new function conn_release().

This new function is now called via session_free() and
session_accept_fd(). It will also be reused on delete server to
proactively close idle connections.