How long the worker waits for the server to answer an ack probe ()
before presuming the socket dead. Probes are sent from the sweep when an
SM-enabled stream has been quiet for a full PING_INTERVAL, and when a
tab (re)joins the established session. Without them, a silently dead
socket (NAT timeout, sleep/wake) is only discovered by the OS's TCP
timeout, which can take many minutes.
How long the worker waits for the server to answer an ack probe (