change generateNRandomKV to generateNKV, add generateNKVwithDepth

@philips Hi, I think you want to generate different keys instead of same keys here, so I changed 'n' to 'i' in generateNRandomKV(), besides since it doesn't actually generate random keys, so I changed the func's name to generateNKV. Also I want to do some bench on snapshot, so I wrote a wrapper to generate keys with specified depth.