Simplify NioPool garbage collector.
The GC now simply scans the directory and identifies index and binary files to keep, based on last modification time alone. It no longer walks the revision history, so missing revisions won't be a problem. The "Keep N revisions" feature was removed, as it makes no sense. A user might create 100 revisions a second, or none in a year. Revisions should only be considered to allow MVCC, not be a 'history' of changes that one might want to preserve. If so, GC must be disabled completely.