qnetd: Improve dead peer detection

qnetd: Improve dead peer detection

Previously dead peer detection timer was scheduled every dpd_interval,
added dpd_interval to all of the clients timestamp and if timestamp was
larger than client hearbeat interval * 1.2 then check if client sent
some message. If so, flag was reset.

This method was source of number of problems so instead different method
is now used.

Every single client has its own timer with timeout based on
(configurable) dpd_interval_coefficient and multiplied with
client heartbeat timeout. When message is received from client timer is
rescheduled. When timer callback is called (= client doesn't sent
message during timeout) then client is disconnected.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>