Refactor the entities' stats API to simplify using it and reduce the
risk of running into concurrency issues at the call sites. Further
simplify the stats code by de-spaghetti-ing the logic and reducing
duplicate code.
`ContainerStats` now returns a data channel and an error. If the error
is nil, callers can read from the channel.
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>