Buffer stats in influxdb driver before writing to the Db. 

The buffering duration is configurable via a flag and is set to 60 seconds by default.
This new flag replaces exiting flag "history_duration" since they both are inter-dependent.
The storage driver test code has been re-factored a bit to help test the new buffering
feature and will be future proof.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Since we buffer data in influxdb driver based on time duration, we have to hold stats in the cache for that duration. The number of samples can vary based on the housekeeping duration. To the end user, it is easier to think it terms of duration rather than number of samples * housekeeping duration. 
The cache is only used by the UI and so holding just 60 seconds of data might be enough even if the buffering duration for influxdb is higher.