chimera: add ".(get)(filename)(checksum)" command

Adds command

".(get)(filename)(checksum)"
OR
".(get)(filename)(checksums)"

which returns a comma-delimited list of type:value pairs for all checksums stored in the database.

Note that the FileSystemProvider API was extended with a method to retrieve a set of org.dcache.util.Checksum objects, and the FsSqlDriver similarly was provided with the necessary wrapper call to the database.

Testing: Works on both nfs3 and fns4.1.

Target: master
Request: 2.7
Patch: http://rb.dcache.org/r/6405
Requires-book: yes
Requires-notes: yes
Depends-on: http://rb.dcache.org/r/6404
Acked-by: Tigran

BOOK:
RELEASE NOTES:

A new NFS dot-command has been added to retrieve all the existing checksums for a given file (see body of description).