initial exchangelayer and trader cleanup:

- Makes it easier to add support for new exchange/api with differing api
  features.
- Remove AssetTradeMin leftovers.
- Only cancel previous order if we're placing a new order.
- Cleanup some unnecessary waits, set 2s APIWait for OKCoin which is
  their exact limit.
docs: add informative Contributing markdown
loggerdb and simulator inline with exchangelayer