net: dns: Split long resolving to smaller pieces

If getaddrinfo() is called with a long DNS timeout, then split
the timeout to smaller pieces with exponential backoff. Reason
for this is that if a DNS query is lost, then we do not need
to wait for a long time to find it out.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>