Factor some client-related functions into client-info.lisp.
Revamp client-info.

Document the expected format of the file input to LOAD-CLIENT-INFO,
and change how FILE-INFO instances are structured.

This introduces file information (octet sizes, cryptographic digests)
meant to make it easy to validate client info files, and avoid
downloding them in the first place if possib.e
More client-file-info cleanups.

  - Add print-object method

  - Remove older code for fetching metadata

  - Error-checking for missing plist data