Handle locked pages more robustly (Fixes issue #1462)

Memory locks do not stack, that is, pages which have been locked several times by calls to mlock()
will be unlocked by a single call to munlock(). This can result in keying material ending up in swap when
those functions are used naively. In this commit a class "LockedPageManager" is added
that simulates stacking memory locks by keeping a counter per page.