Use Timeout Conn wrapper to set read deadline for downloading layer

Docker-DCO-1.1-Signed-off-by: Derek <crq@kernel.org> (github: crquan)
Added a new method cgroups.GetStats() which will return a cgroups.Stats object which will contain all the available cgroup Stats.
Remove old Stats interface in libcontainers cgroups package.
Changed Stats to use unit64 instead of int64 to prevent integer overflow issues.
Updated unit tests.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Merge pull request #6096 from vishh/stats_strongtype

Replaced fs.Stats() interface with fs.GetStats() which returns a cgroups.Stats strong type
Merge pull request #6048 from crquan/use-timeout-conn

Use Timeout Conn wrapper to set read deadline for downloading layer