MINOR: proto_reverse_connect: use connect timeout

MINOR: proto_reverse_connect: use connect timeout

Use backend connect timeout when a new connection is instantiated for
rhttp. This ensures that if connect operation fails after a certain
delay, reverse_connect listener task is woken up. This allows to free
the current connection and retry a new connect.

As a consequence of this change, rev_process() may be woken up even if
connection is not reported with CO_FL_ERROR. This happens if timeout
fired before any network reported issue. Connection freeing is adjusted
as in this case MUX instance is already allocated. Use destroy callback
to release MUX context prior to the connection itself.

This patch is really useful as a side measure for a haproxy bug
impacting connect with SSL for both backend connections and active
reverse connect. This is caused by the delayed allocation of MUX
allocation. Asynchronous connect error detected at the socket layer is
not notified to upper layers. Currently, only connect timeout allows to
release this failed connection.