Scaler Webui should show object storage byte size

[sharpener6]

We should have some heartbeat message from object storage, that will tell scheduler how many bytes it holds currently, and scaler top and webui should show this size

[gxuu]

@sharpener6 This issue was discussed before. The resolution back then was, instead of sending heartbeats, that Object Storage Server provides a new method for user to pull information actively. Here's the issue and PR related to the discussion.

#362
#364