Copy global model when retrieving it from local tablet

[wynot12]

In PS-collocation mode, workers retrieve the global model from servers including its local server.
In that case, ET's get API returns the original objects stored in table.
It becomes problem when the returned objects mutate in background.
Workers may see the intermittent state of model values, which violate the format.

In summary, workers need to use the copy of mutable model, which can be updated by other (remote) worker threads.