net: lib: sntp_simple: Issue multiple requests with backoff

SNTP is UDP-based protocol, and thus not reliable. Previously,
high-level aka "simple" SNTP just issues a single request via
the low-level SNTP API. Instead, send multiple requests, starting
with a small timeout, and exponential backoff, repeated within
timeout specified by user in call to sntp_simple().

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