Sync system clock with SNTP server

[sei0o]

applications/tests/test_ntp/src/lib.rs contains the test code.

Here we introduce some structs:

• NtpTimestamp represents the timestamp defined in the specifications (e.g. RFC4330). SystemTime represents how long has it passed since unix epoch in nanoseconds. It can be converted to NtpTimestamp and vice versa.
• SystemClock provides the interface for NTP daemons and managing the system clock. SystemClock::now() returns a SystemTime.