Make NetworkStats a pointer to help the callers ignore that stats if it cannot be returned for any reason.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Do not mark any network stats as 'omitempty' since the value '0' is legal for those stats.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Merge pull request #119 from vishh/fix_network_stats

Minor fixes to network stats