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)
There is also a nice flag.Duration which can accept human-friendly values