Factor out data comparator for storage tests.
For influxdb, we are planning to export lesser data. Having a separate
comparator will let us reuse the same tests.

Docker-DCO-1.1-Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Merge pull request #168 from rjnagal/master

Factor out data comparator for storage tests.