Feature Request: When switching configs to new pool, send "old shares" received after the switch to the old pool and "new shares" to new pool
When switching coins on xmrig-proxy I am noticing quite a number of rejected shares:

![image](https://user-images.githubusercontent.com/35117940/37121918-a4fb4ae6-222c-11e8-947d-ec393ae89dfb.png)

This happens with or without diff tuning on the pool end. It would be awesome if xmrig-proxy could send old shares received to the old pool and new ones to the new pool.