cache: Store metadata size and checksum.

cache: Store metadata size and checksum.

Refactor the recent metadata-reading optimisation patches.

Remove the recently-added cache fields from struct labeller
and struct format_instance.

Instead, introduce struct lvmcache_vgsummary to wrap the VG information
that lvmcache holds and add the metadata size and checksum to it.

Allow this VG summary information to be looked up by metadata size +
checksum.  Adjust the debug log messages to make it clear when this
shortcut has been successful.

(This changes the optimisation slightly, and might be extendable
further.)

Add struct cached_vg_fmtdata to format-specific vg_read calls to
preserve state alongside the VG across separate calls and indicate
if the details supplied match, avoiding the need to read and
process the VG metadata again.