Use sync.Pool to use &item{}

[choleraehyq]

benchmark                                    old ns/op     new ns/op     delta
BenchmarkConcurrentCacheSetNeverEvict-12     96.3          127           +31.88%

benchmark                                    old allocs     new allocs     delta
BenchmarkConcurrentCacheSetNeverEvict-12     3              2              -33.33%

benchmark                                    old bytes     new bytes     delta
BenchmarkConcurrentCacheSetNeverEvict-12     110           108           -1.82%

---

This change is 

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}