BUG/MEDIUM: mux-spop: do not enforce the timeout on connections with streams

BUG/MEDIUM: mux-spop: do not enforce the timeout on connections with streams

spop_conn_update_timeout() leaves the connection task unarmed while stream
connectors are attached, since the timeouts are then enforced by the upper
layer. But spop_attach() does not touch the task, so a connection which got
a timeout while it had no stream, for instance because of pending output
data or because of the default timeout applied to connections without
stream, keeps it once a new stream is attached. When it strikes,
spop_timeout_task() kills the connection and aborts the streams attached
to it, while nothing indicates that the agent is not responding.

As done for the H2 mux by commit c2ea47fb1 ("BUG/MEDIUM: mux-h2: do not
enforce timeout on long connections") and for the FCGI mux by commit
6e3bad2f3 ("BUG/MEDIUM: mux-fcgi: do not enforce the timeout on connections
with streams"), spop_timeout_task() now does not kill the connection when
stream connectors are still attached, it only disarms the timeout.

This should be backported to 3.1.