RefCell to reuse buffer during insertion when only have Shared Reference (&T)

[GordonYuanyc]

• Location: CountL2HH
• Explanation:
  • CountL2HH have a function that perform "insertion and query"
  • Used by UnivMon insertion, which will potentially allocate small buffer repeatedly (query a CS will need get median, which usually requires a Vec or something similar)
• Scope: UnivMon only at this moment
• Possible drawback: concurrency scaling