Fold CMS/CS to regular CMS/CS and possible merge-ability

[GordonYuanyc]

• When the fold_cms and fold_cs is not sparse, the size usage of fold sketch is larger than regular sketch (where counter is just a number, i.e., i64)
• A function to convert a fold sketch to a regular sketch when fold sketch is no longer sparse to save space
• Merge a fold sketch to a regular sketch may be necessary, when the window list have mixed sketch
  • This will be helpful if the conversion between fold sketch and regular sketch is effective