Modify exception headers

Two modes

### Default

Remove 

 * `NServiceBus.ExceptionInfo.InnerExceptionType`
 * `NServiceBus.ExceptionInfo.HelpLink`
 * `NServiceBus.ExceptionInfo.Source`

Replace the value written to `NServiceBus.ExceptionInfo.StackTrace` with `Exception.ToString`

### Legacy

Force exception headers back to `legacy mode` ie keep original headers and use `Exception.StackTrace` in `NServiceBus.ExceptionInfo.StackTrace`.

Enabled by setting the `NServiceBus/AssemblyScanning/UseLegacyExceptionHeaders` AppSetting to `true`.