samples: sntp_client: Elaborate sample to workable state

1. Output what steps the app performs, so it doesn't look to user
that it simply hanged.
2. Don't use infinite timeouts, because that will hang.
3. Clearly note which requests are for IPv4 vs IPv6 server.
4. Define IPv4 gateway. This sample is configured to run against
SNTP on local Linux host, but standard distros (e.g. Ubuntu) don't
run SNTP server by default, so usual outcome for running this sample
will be timeout. A realistic way to get successful output would be
to run it against a server on the Internet, for what a gateway is
required.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>