Cache and share generated VG structs

Cache and share generated VG structs

Extend vginfo cache with cached VG structure. So if the same metadata
are use, skip mda decoding in the case, the same data are in use.
This helps for operations like activation of all LVs in one VG,
where same data were decoded giving the same output result.

Patch adds 1-to-1 connection between volume_group and lvmcache_vginfo.