fix events subscription from other phases

[paulormart]

With this change System events like staking::events::EraPaid, session::events::NewSession, grandpa::events::NewAuthorities can be subscribed, otherwise they are currently being skipped using the pseudo code below:

let client = self.client();
let sub = client.rpc().subscribe_finalized_events().await?;
let decoder = client.events_decoder();
let mut sub = EventSubscription::<DefaultConfig>::new(sub, &decoder);
sub.filter_event::<node_runtime::staking::events::EraPaid>();
while let Some(result) = sub.next().await {
    if let Ok(raw) = result {
        match node_runtime::staking::events::EraPaid::decode(&mut &raw.data[..]) {
            Ok(event) => info!("Successfully decoded event {:?}", event),
            Err(e) => error!("Error decoding event {:?}", e),
        }
    }
}

[paulormart]

reference to pull request #294

[sander2]

This is a duplicate of #283, but this will definitely be good to merge into the open PR

[paulormart]

@ascjones master branch targeted

[ascjones]

Closing as duplicate of #283