Keep cost models history and notify postgres

[gusinacio]

We want to check receipt values to see if they are the minimum receipt value by our cost models.

The problem is that the current update cost model function overwrites the old model and in that particular moment, a gateway could have failed requests because it's still using the older value.

We want to keep track of the history of cost-models so indexer-service can accept both the actual model AND the last model for a defined amount of time (i.e: 2 mins). We also want to be notified via pg-notify so indexer-service can update as soon as possible instead of polling the database.

The following PR depends on this: graphprotocol/indexer-rs#153