Also remove `MaxUpstreamConnsFlag` const as it's no longer referenced in more than one place and to make things more consistent with how the other flags are referenced.
Allows defining a limit to the number of connections that can be
established with the upstream DNS host.
If left unset, there may be situations where connections fail to
establish, which causes the Transport to create an influx of connections
causing upstream to throttle our requests and triggering a runaway
effect resulting in high CPU usage. See https://github.com/cloudflare/cloudflared/issues/91