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travis-pro

implement historic price estimates (#1138)

This PR adds initial support for computing historic prices. In a follow up PR, I will ensure that the `OrderbookReading` implementation doesn't re-query the node for events if the batch ID is sufficiently in the past (to speed up the request a bit).

Closes #967 

### Test Plan

Run the price estimator and check historic prices - note that it is currently not an error checking future batches.
```
$ curl -s 'http://localhost:8080/api/v1/markets/1-7/estimated-best-ask-price?atoms=true' | jq
230.43986139024014
$ curl -s 'http://localhost:8080/api/v1/markets/1-7/estimated-best-ask-price?atoms=true&batchId=5290000' | jq
150.08262567963578
$ curl -s 'http://localhost:8080/api/v1/markets/1-7/estimated-best-ask-price?atoms=true&batchId=9999999' | jq
230.43986139024014
```

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