Using sync.Pool for creating element values

[zekroTJA]

Currently, each time a new element is set as value to the map, a new element object is allocated on the heap and the pointer of it then set as value to the map.

This causes a lot of allocations when setting new key-value pairs. This might be bypassed by using a sync.Pool to create and cache element objects so they can be re-used after a value was deleted from the map, which often happens when values are expired.

See this post as "template" on how to implement and use sync.Pool:
https://www.akshaydeo.com/blog/2017/12/23/How-did-I-improve-latency-by-700-percent-using-syncPool