Logging Filter: refactored and cleaned up the formatting of results + a few XML comments
Added crude implementation of Fdb.System.EstimateCountAsync(...) that tries to count the number a keys under a given subspace

- Uses a key selector with a window size to scan through the range, so it does not need to read all the keys and values in the range.
Tweaked the 'dir' command to count the number of keys under each directory subspace.