gnosis / dex-subgraph / 375

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travis-pro

Global stats (#145)

This PR adds a stats entity which allows us to record metrics like total volume, fees, number of trades, batches, etc. I got inspired by our top level dune metrics which we should now be able to answer e.g. for xdai using the graph.

<img width="1330" alt="Screenshot 2020-11-05 at 19 51 09" src="https://user-images.githubusercontent.com/1200333/98283664-44892880-1fa0-11eb-96d8-85c61938470c.png">


### Test Plan

Added unit and e2e tests

40 of 40 new or added lines in 4 files covered. (100.0%)

446 of 462 relevant lines covered (96.54%)

8.63 hits per line