Rewrite storage interface + implementations to be async-compatible

[JWCook]

Currently, all the storage classes use a dict-like interface (collections.abc.MutableMapping). Unfortunately, there is no syntax support in python for async dict operations, e.g.:

class AsyncDict(MutableMapping):
    async def __getitem__(self, key):
        # ...
    async def __setitem__(self, key, value):
        # ...

my_dict = AsyncDict()
await my_dict['key'] = 'value'
await my_dict['key']

So, this will require a new base storage interface that uses regular methods rather than dict operations.

Ideally, it would be good to merge the storage base class with BaseCache, if there is an elegant way to do it. Currently, they are separate so that BaseCache can have separate references to a key -> response collection and a redirect_key -> key collection.