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.
Sure. I'll change it. Since this function will be removed in the next PR. So it doesn't matter either it returns an error or panics.