Deprecate using cache specific constructors

[argaen]

Cache class was introduced as a proxy for instantiating the different backends. There should be only one way of doing things and Cache is the preferred way of doing that so will deprecate the others

[padraic-shafer]

This should probably also apply to creating the cache inside the decorator @cached. It looks like this might have been overlooked.

https://github.com/aio-libs/aiocache/blob/02782c238aa8a6ca238ef938b3c70e017dc5a2ea/aiocache/decorators.py#L207-L208

PR #568 was submitted.

[Dreamsorcerer]

This is an open issue, so doesn't look like anybody has deprecated anything yet.

[Dreamsorcerer]

I'm not sure how well this'll play with static typing, so I suspect we'll have to change this again as we introduce typing to the library.

It seems slightly odd to me that we instantiate a wrapper class and pass the implementation class as an argument, rather than just using it directly. i.e. Why do Cache(Cache.MEMORY) and not just Cache.MEMORY() or SimpleMemoryCache()?