Add query for only existing checksums

This allows user to query checksum without triggering checksum calculation. This might be useful in case user asks using Want-Digest header for multiple checksums (e.g. "Want-Digest: adler32;md5;crc32"), because could trigger calculation of multiple checksum at same time. Fortunately there is a bug in code parsing Want-Digest header which currently prevent such behavior and requests for multiple checksum types are effectively ignored.