Most of the sleeps related to gossiping are based on `ring_delay`,
which is configurable and can be set to lower value e.g. during tests.
But for some reason there was one case where we slept for a hardcoded
value, `service::load_broadcaster::BROADCAST_INTERVAL` - 60 seconds.
Use `2 * get_ring_delay()` instead. With the default value of
`ring_delay` (30 seconds) this will give the same behavior.
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