impl the mysql version ExceptionStore to support view the exception …

[yuzukwok]

…store in the mysql database

[NickCraver]

I'm not against adding this (though the dependency has concerns), but I need to get exceptions working properly on SQL server first. I've just put in groups and am fixing any issues there.

This won't be mergeable as-is since so much has to change, but a few comments I'll go ahead and add for the current state:

• "mssql" is a bad string to pass around everywhere, this should be an enum, e.g. StoreType.MSSQL and StoreType.MySQL
• The formatting is all over the place - which IDE are you using? I may be able to help there
• The MySQL data package doesn't actually do async. It's all synchronous and thread-eating underneath. We'd have to use another provider (there are some decent github options coming along with .NET Core - which also shouldn't block on that front)
• Settings: should also use the enum

[yuzukwok]

You're right. I will try to fix the issues.

[NickCraver]

Closing out to cleanup - unmergeable due to comments above.