Multibid scoring context sharing

[corywalker]

We're testing multibid (#1048) support. I'm looking at some traces and if multiple ads need to be scored from an IG, I see one context getting created for each ad. I'm hoping at the least it could make sense to share contexts across ads in the same IG. If we can share across an even broader scope than IG that could also help with efficiency (for example in the same joining-origin, similar to groupByOrigin).