Return pointer instead of value in SeldonApiClient methods

[RafalSkolasinski]

Methods defined by SeldonApiClient interface in executor return payload.SeldonPayloads via value not reference.

Is this a potential bottleneck? Could we gain by returning pointers there?

Conscious this is for consistency across all functions above, but why are we 
returning `payload.ModelMetadata` by value instead of reference/pointer? 
It seems like it would be quite inefficient to copy all the values that are 
returned by the client

_Originally posted by @axsaucedo in #2005 (comment)

[glindsell]

payload.SeldonPayload is actually an interface which can hold either a struct or a pointer to a struct.
payload.ModelMetadata however, is a struct so a pointer should be returned.

[RafalSkolasinski]

This is perfect @glindsell, thanks for the insight! I had to forgot about that Go nuance :)

[RafalSkolasinski]

Going down on Golang good practices I now wonder if it is best to have

1. SeldonApiClient.Metadata method that returns payload.SeldonPayload storing raw payload and SeldonApiClient.ModelMetadata that returns model metadata decoded to payload.ModelMetadata struct

or

2. Have one method in SeldonApiClient that returns payload.SeldonPayload and extend payload.SeldonPayload interface to have a method that decodes payload to ModelMetadata struct.

[ukclivecox]

I think the SeldonPayload is meant to be very generic and not really get into the machine learning specifics. So I'd suggest not unless a good reason.

[RafalSkolasinski]

I think the SeldonPayload is meant to be very generic and not really get into the machine learning specifics. So I'd suggest not unless a good reason.

Ok, that basically cross away option 2 meaning that what is currently in metadata PR is right up to the return value instead of pointer for optimisation.