Correct information in samples

The CPU usage in ContainerStats is cumulative usage, not instantaneous usage. The makes the samples useless because randomly sampling cumulative CPU usage is not helpful for the user.

In this PR, I made the following changes:

- Add a new strcut called ContainerStatsSample, which is used to store a sample in which the CPU usage is instantaneous
- To generate a sample, we must have two ContainerStats object so that we could calculate the diff of cumulative CPU usage.
- Make the corresponding changes in container package.
- Unit tests.
No, it does not solve the time problem.

Here is what I want to test in ``TestSampleCpuUsage()``:

Given N+1 observations, there should be N samples and each of them should have duration of one second. Using real time will not guarantee the sample duration.

One possible solution is to add another container handler decorator which only set the stats' time. This makes the code more clean because ``statsSummaryContainerHandlerWrapper`` will only do stats. (Right now, it also sets timestamp.) The downside of this approach is that adding another layer for container handler may affect performance.