Update the `usage()` method of containerizer to set resource limits in the `ResourceStatistics` protobuf message

In the `ResourceStatistics` protobuf message, there are a couple of issues:

1. There are already `cpu_limit` and `mem_limit_bytes` fields, but they are actually CPU & memory requests when resources limits are specified for a task.
2. There is already `mem_soft_limit_bytes` field, but this field seems not set anywhere.

So we need to update this protobuf message and also the related containerizer code which set the fields of this protobuf message.