Add remove method to return Option<V> for unsync cache (v0.11.x branch)

[tatsuya6502]

On #43, @JokerXiL said:

With current implementation of unsync cache, it is possible for the invalidate method to return previous cached value.

I modified it and put it as new remove method.

In order to merge the stuff for #43 into the current active branch v0.11.x, I created a new branch from v0.11.x branch and ran git cherry-pick 7cad7fd44afd61a09df18b3c0b7bb4eb63e14faf to take the commit.

[tatsuya6502]

Merging.