Give chance to reuse connection for sequential CMDs

[argaen]

In codes like

            await redis_cache.set("hi", "value")
            await redis_cache.get("hi")
            await redis_cache.delete("hi")

backend acquires and releases the connection from the pool every time which adds extra cost. In aioredis, reusing the same connection or acquiring (even when its the same) almost doubles the time.

Usage could be some like:

with redis_cache as conn:
    await conn.set()
    ...

With memory it doesn't make sense but to keep compatibility it will return itself. Its also possible to do it with memcached (although haven't checked the difference).

[argaen]

Couple of problems:

• Due to bad design of memcached its not possible to do that. When calling memcached CMD commands, they retrieve the connection everytime (https://github.com/aio-libs/aiomcache/blob/master/aiomcache/client.py#L13). Code should be changed so a connection can be passed
• With redis we can retrieve the conn object, but still we want to apply the namespacing, plugin calls, serializers, etc. So its not easy as thought

Also in case of redis, the pool should be initialized in the constructor so we avoid calling the get_connection when the pool is None:

    @pytest.mark.asyncio
    async def test_single_connection(self, cache):
>       async with cache.get_connection() as conn:

tests/acceptance/test_base.py:133: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
aiocache/base.py:457: in __aenter__
    self._conn = await self._pool.acquire()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = RedisCache (127.0.0.1:6379)

    async def acquire(self):
>       return await self._pool.acquire()
E       AttributeError: 'NoneType' object has no attribute 'acquire'

https://github.com/argaen/aiocache/tree/feature_%23225/reuse_connections