Persist paper exchange state

[dazito]

It would be a nice to have feature for the paper exchanges to persist their state.

When running paper mode, if we restart the bot we will lose any entry trades on the paper exchange. Because of arbitrade-state json file (where we persist the entry trades), when the bot starts up it will load the content of arbitrade-state json file. If there are entry trades, after a while the bot will try to exit the trade but the paper exchange will not recognize the short that we are trying to close because it was lost during the restart.