mem_cache_store requires dalli, so only accept dalli/client

:mem_cache_store require dalli, rescue Dalli exceptions, and follow Dalli API.
Memcached gem, for instance, doesnt work anymore, as the API are different.

As we already require one client, we should make sure that client works, and not accept others, and if someone wants to use another memcache client they can write their own store adapter.
Raise if MemCacheStore doenst receive a Dalli obj

:mem_cache_store should receive a list of hosts or a dalli client,
otherwise raise it.
Also adding a changelog.
Merge pull request #13583 from arthurnn/mem_cache_store_fix

mem_cache_store requires dalli, so only accept dalli/client