Attach a model name to the serialized GAME model

[XianXing]

The Ads team made a suggestion to us, that we may want to add something like a model name to the serialized GAME model, which might be helpful if multiple GAME models are trained offline and served online at the same time.

I guess one thing we can do is to add a field called modelName to each Avro record in addition to the field modelClass we added recently in PR #123.

However, we may need to disambiguate the existing modelId field and the to-be-added modelName field, or we need better names for both of them. For example, rename modelId to effectId, because essentially what modelId current does is to store the fixed/random effect id associated with that particular fixed/random effect model, like the user id (if it's a per-user random effect model) or the item id (if it's a per-item random effect model).

[fastier-li]

I wrote a model format converter for another LI system recently and included a model name in the output file. I'll see how to "finish that up" by surfacing the name more.