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.

aiocache/aiocache/decorators.py

Lines 207 to 208 in 02782c2

	def _get_cache(cache=Cache.MEMORY, serializer=None, plugins=None, **cache_kwargs):
	return cache(serializer=serializer, plugins=plugins, **cache_kwargs)

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()?