in memory event sourcing to mirror exchange transaction

[asmodehn]

We need to retrieve transactions from the exchanges.
But we want it light weight yet a first step towards using event sourcing (hoping to get some benefits out of a minimal implementation of it)
Ref: https://github.com/commanded/commanded

[asmodehn]

Note this should be done in the :xest app.
The connector apps are already complex enough without this.

[asmodehn]

We might want to store the original data from the exchange however, not the dumb down data (lowest common denominator between all exchanges).

Not sure how this is doable without doing it in the connector... or exposing the complex data to the web app.
This needs further reflection...
Maybe the xest app should provide some protocol to allow the connectors to "add" event sourcing functionality, when used with xest, but keep xest somehow agnostic to the event datastructure stored.

[asmodehn]

• This is required for MultiSymbol Trades view for Binance #33
• This requires Review cache / clock design #29 to be done first. The local cache needs a way to choose to use the cache, or retrieve remote, based on data content (the time), and the clock. It is a tricky thing that should be done right as a first step.