Websockets/pub-sub RPCs should be expanded to allow RPC clients to submit extrinsics and get full lifetime notifications of them.

• -> author_submitExtrinsic(xt: Vec<u8>) -> Result<ExtrinsicHash, Error>
• <- author_extrinsicUpdate(xt: ExtrinsicHash, status: Status)

Error may be:

• InvalidFormat (i.e. it's plain old invalid and will never become valid)
• Dead (i.e. it was once valid but has now become invalid)
• Immature (i.e. it's currently invalid and while it may become valid at some point, that's too far ahead to care about)
• PoolFull (i.e. there's no room at the inn)
• AlreadyKnown If we already know about it but yet it's valid for inclusion, then it's not an error - we carry on as before and trace the pre-existing extrinsic instead.

Status may be:

• Finalised(BlockHash) (it's finalised and all is well)
• Usurped(ExtrinsicHash) (some state change (perhaps another extrinsic was included) rendered this extrinsic invalid)
• Broadcast(Vec<PeerId>) (it has been broadcast to the given peers) version 2.0 only