keep the key never overrides

[membphis]

never overrides the (least recently used) unexpired items in the store when running out of storage in the shared memory zone

[agentzh]

I think it's better to expose an option to let the user decide whether to use add or safe_add. Not every use case requires such strictness, for example, in the case of cache locks.

[membphis]

@agentzh please take a look at first, add the test case later.