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.
The storage driver test code has been refactored 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)