TRQ-2761 force update flag

Both approaches are implemented (see details in the task comments):
1. First 4 commits: 6322bd4, d55bf28, d92600b and 137e509.
Don't set last timestamp to zero when status send needs to be forced. Adjust it's value to force immediate update sending instead: last status timestamp = time now - timestamp send interval.
2. Last 3 commits: 48ae655, 48050e6 and b6445cc.
Introduce a new global "force update" flag and don't touch the actual timestamps. In this case the timestamps values will always represent the actual timestamp and the flag will be set/cleared when needed.
If you're agree with the second approach you just can merge this request completely.