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.
Ah, Sure. Just did it.

About line 214: In line 64, the time is stored in microseconds. It is the finest granularity allowed by influxdb (or its client API?) The time retrieved is, however, in millisecond.