first step for a faster scan of network drives

We only run through a network collection once in
the first scan to determine the number of files.
The possible number of images and albums is now
taken from the database.
This can be a little less precise under certain
circumstances, we will test it.
CCBUGS: 405235
CCBUGS: 437771
CCBUGS: 431037
CCBUGS: 265241