Whitespace again.
Refactored out bucket_stats.go file.
Generic StatsSnapshot interface, to Statish interface is generic.

This is in preparation for having server-level stats in addition to
bucket-level stats.
Moved bucket related stats to bucket_stats.go file.

And got rid of stats_util.go, as generic stats stuff is now in
stats.go file.
Refactored out vbucket_mutation.go file.