Parity-ethereum: add newPendingTransactions to subscriptions

[tzapu]

_Before filing a new issue, please provide the following information._

I'm running:

• Parity version: Parity//v1.7.0-beta-5f2cabd-20170727/x86_64-linux-gnu/rustc1.18.0
• Operating system: Linux

_Your issue description goes here below. Try to include actual vs. expected behavior and steps to reproduce the issue._

---

actual
when trying to subscribe for new pending transactions you get a not implemented message

> {"method":"eth_subscribe","params":["newPendingTransactions"],"id":1,"jsonrpc":"2.0"}
< {"jsonrpc":"2.0","error":{"code":-32000,"message":"This request is not implemented yet. Please create an issue on Github repo."},"id":1}
>

expected
to be able to subscribe for new pending transactions

thank you

[artemii235]

Hello. Please note that for compatibility with web3.js this subscription should be called just pendingTransactions: https://web3js.readthedocs.io/en/1.0/web3-eth-subscribe.html#subscribe-pendingtransactions. Thanks!

[tomusdrw]

Will probably be implemented together with transaction queue refactoring --> First part: https://github.com/paritytech/parity/pull/6994

[k06a]

web3.eth.subscribe('pendingTransactions', function(error, result) {
    console.log(error);
})
.on('data', function(txData){
    web3.eth.getTransaction(txData).then(console.log);
});

=>

This request is not implemented yet. Please create an issue on Github repo.

[tzapu]

this works lovely 👍