gossiper: Increase echo message timeout
gossiper: Increase echo message timeout

Gossip echo message is used to confirm a node is up. In a heavily loaded
slow cluster, a node might take a long time to receive a heart beat
update, then the node uses the echo message to confirm the peer node is
really up.

If the echo message timeout too early, the peer node will not be marked
as up. This is bad because a live node is marked as down and this could
happen on multiple nodes in the cluster which causes cluster wide
unavailability issue. In order to prevent multiple nodes to marked as
down, it is better to be conservative and less restrictive on echo
message timeout.

Note, echo message is not used to detect a node down. Increasing the
echo timeout does not have any impact on marking a node down in a timely
manner.

Refs: #7197

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla