resolver: Send RS before RDNSS lifetime expires

resolver: Send RS before RDNSS lifetime expires

Implemented feature from RFC 6106 section
'5.1. Recursive DNS Server Option':
	"Lifetime      32-bit unsigned integer.
	...
	Hosts MAY send a Router Solicitation to ensure
	the RDNSS information is fresh before the interval expires."

Host will send RS when a certain threshold of RDNSS lifetime is reached.
Values which can be adjusted:
- lifetime threshold - set to 80% from lifetime
- number of retries in case RA is not received - set to 0
- time between retries, in case RA is not received - set to 3 seconds