FOCUS-4102, FOCUS-4441 Fix for defects 1 through 4.
For defect 2 we crawl just once on items whose visibility is off and
then create DELETE entries. For 3 we propagate the ACLs from a list
towards its attachments, finally for defect 4 we use Tika to detect if
the content of a document is encrypted and if it is we remove it.