Refactor aggregators list and add gas limit

I have questions about validating this code before anyone reviews this PR. I am unsure of the procedure to test those changes locally.

make build gives no error, and make test passes.

1. Son of Odin suggested we refactor the aggregators list to use the _current pattern to improve git diffs.
2. I suggest per-contract gas unit limits.

I combined those two as there should be no logic change from this PR.