Add MGet or Multiple Get to improve read performance

[Planxnx]

I've performance issues when my cache has heavy read and write load in the same time.
I propose an approch like Reduce mutex blocking while get from multiple keys (like redis MGET ) Or use RW.Mutex while Get() instead.

[gozeloglu]

Hey @swithek. What about this issue? Should it be added as a new feature? If yes, I'd like to work on it.

[swithek]

@gozeloglu I'm not sure it's possible to implement the suggested changes, at least not all of them. I go into more detail here: #105 (comment)

[kecci]

bump for my discussion also: #160