Enhancement - Scheduled MongoDB prune

[shr00mie]

Enhancement:

set cron or manual command to prune mongo for trade data older than c.days per conf.js. as that can grow rather large if keeping track of multiple exchanges/coins for 30 days of trade data each. if not backtesting further than c.days, no reason to keep data older than backtest duration.

[Fredrik81]

would be nice if this can be in the zenbot code to do this prune and keep it out of cron in order to keep simplicity for the user and also think it will be easier for the docker uses just to change config file and not cron file?