We sleep storage_service_ring_delay until we abort due to failing to
talk to a seed node. We should retry sending GossipDigestSyn message,
instead of sending it once.
With this, we can start the seed node and normal node in a script like
below, without any sleep between.
./scylla --listen-address 127.0.0.1
./scylla --listen-address 127.0.0.2
This is useful for testing.