Initialize-ledger retrieve exchange ledger about gryphon

[garethdmm]

A ledger importer is a great idea. It would also be nice to have since currently users can't run with order_audit audit types enabled initially, since their recent order history exists outside the db.

To start out, one could do a survey of the integrated exchanges and see for which ones this is possible. I know some exchanges will only give full history through a .csv download, so might not be accessible through the API. Others will give trades, but maybe not transactions, and definitely not orders.

This probably wouldn't be a full replacement for the current logic of the initialize_ledger script. Some things are still going to be down to the user to not mess up their ledger (and there are safeguards in place in that script for not overwriting an existing ledger). But I could see them working together where the exchange allows it for sure.