Write only storage: Part 1

This is the first step towards write-only storage driver. In this PR, I added a backend to the in-memory storage, removed cache package, disable ``Samples()`` and ``Percentiles()`` in influxdb and bigquery storage driver.

I think there will be two more PRs which will make the storage driver write only and support batch writes.
Yes. Once this PR merged, cAdvisor will never read from a backend storage. Currently, ``RecentStats()`` only used for testing purpose. I think I will move it into some test-only package like ``StatEq()`` later.