Change nextInstance() to permit retrieval too, use it for bitmap alloc

Change nextInstance() to permit retrieval too, use it for bitmap alloc
- The "was this verified already" bitmap uses header instance numbers
  which are monotonically increasing and can be much much larger than
  the number of actually installed headers. Grab the current instance
  number for a better idea how much we'll at least need.
- Throw out the db stats based dbiNumKeys(), this doesn't have the
  kind of performance penalty that stats have and suits our purposes
  better anyway