Enhance the is_ready() implementation on LagRegister

[detro]

Right now, it looks like this:

#[async_trait]
impl Awaitable for LagRegister {
    async fn is_ready(&self) -> bool {
        self.lag_by_group.read().await.len() > 0
    }
}

This is pretty "weak" as readyness-check.

Something better would be to check that the registry has reached as "stable" number of groups with a stable number of registered lags against it. That might require tracking changes over multiple checks.

Ideas are welcome.