Fix storage driver buffer duration default value.

The cache will now hold atlest the minimum number of stats required by the UI and more
if the buffer duration is longer than the default.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
If the buffer duration were to be set to 30s, then we will end up querying the db for every second.  We want to cache at least what the UI wants. Once we improve the client side UI code, we can bring down the default or may be not have a default at all.
It might help to make the client and the server agree upon a value using a global constant.