Add public reset 

@arthurnn @fbogsany , cc: @jnormore 

When using web servers which share memory and use copy-on-write, you have to do watch out for things like that https://www.phusionpassenger.com/documentation/Users%20guide%20Nginx.html#_example_1_memcached_connection_sharing_harmful

dalli_store supports `Rails.cache.reset` , but if you want to do this with memcached_store, you have to do `Rails.cache.instance_variable_get(:@data).reset` and it makes me :cry: 

This simple PR makes reset public