perf: add setnx2

[chenyijun266846]

1. add setnx2 to bucket, the differents from setnx are:
   • init newItem after double check
   • use func instead of value to reduce init cost of value
   • return existing
2. add setnx2 to cache, the differents from setnx are:
   • return *Item[T]
   • handle promotables

[karlseguin]

thanks