(FACT-624) Add structured uptime fact

This commit adds a new 'system_uptime' fact, which returns a map including
keys for seconds, hours, days and a human readable uptime.

Also, the existing uptime facts were updated to take their values from this new 
structured fact, rather than resolving them individually.
Unless we directly refer to an included type in the header, including the file in the .cc file is preferred.