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.
@vishh Yes. You can see that I tried to print the time out but still cannot recover the right time with correct precision.

It may be caused by my influxdb instance, which only returns one point when I call "select * from t ...". Still trying to find what's wrong with it.