cache: introduce allocation/cache_metadata_format

cache: introduce allocation/cache_metadata_format

Add new profilable configation setting to let user select
which metadata format of a created cache pool he wish to use.

By default the 'best' available format is autodetected at runtime,
but user may enforce format 1 or 2 ATM.

Code also detects availability for metadata2 supporting cache target.

In case of troubles user may easily Disable usage of this feature
by placing 'metadata2' into global/cache_disabled_features list.