Bluetooth: Mesh: Reschedule dedicated relay adv sets immediately

Reschedule dedicated relay adv sets immediatey after finishing
advertising of the previous relay message. This allows a node to
relay messages as fast as possible when Relay Retransmit Count state
is zero using less relay adv sets.

When Relay Retransmit Count state is non-zero, the delay doesn't do
anything as the time between pushing a relay message to the host and
triggering adv_sent callback will be greater than this delay. In this
case the relay adv set will send a next message immediately.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>