Item8327: Series of fixes and improvements for DBCacheContrib

1. reverted creating subdirectories for subweb cache files
2. only file-based archivists create a file for caching
3. made query parser trivalent: true, false, undef. The latter captures various logical error cases that properly propagate up the parsing tree
4. don't nuke the archivist object although the surrounding cache object still exists. otherwise a fetch tries to derefrence an undef value ... and things die
5. fixed incremental caching: take out non-existing topics
6. added undef checks to prevent invalid dereferencing in various places, e.g. traveling down a from hash
7. added defined predicate useful to find out if a formfield is defined