Rrdcheck new tests

[xmacan]

fix issue#2871

new tests:

• Number of DSs in database is lower than in RRD file/DS name exists in RRA file, but not in database
• RRD file is not writable
• Last update value in RRD file is older than 1 hour
• Database DS name is empty. Local data ID = ...
• DS name exists in DB, but not in RRA file
• ...

@TheWitness - I think I misunderstood your edit start - stop - duration changes. Duration is now 173940, I would expect something around 86400. RRD check on my data is now running longer than before. Or is it a combination of my code and yours?

Please test it