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.
FWIU influxdb's default resolution is milliseconds. In any case, we do not plan to read back data from influxdb correct?