Asynchronous transactions (polling based for now).

[gavofyork]

• Add eth_postTransaction as an async alternative to eth_sendTransaction. Returns a Confirmation promise ID.
• Add eth_checkTransaction, takes a single argument - a Confirmation promise ID and returns
  one of:
  • Transaction hash (signed and submitted).
  • null (still pending, call again later),
  • Zero hash (will never be signed).

[coveralls]

Coverage decreased (-0.1%) to 76.264% when pulling f223199 on async_sendtx into 5ab18d1 on master.

[gavofyork]

Additional web3.js code required to make this work:

web3._extend({
    property: 'eth',
    methods: [
        new web3._extend.Method({
            name: 'postTransaction',
            call: 'eth_postTransaction',
            params: 1,
            inputFormatter: [web3._extend.formatters.inputCallFormatter]
        })
    ]
});

web3._extend({
    property: 'eth',
    methods: [
        new web3._extend.Method({
            name: 'checkTransaction',
            call: 'eth_checkTransaction',
            params: 1
        })
    ]
});

{
    var postTransaction = web3.eth.postTransaction.bind(web3.eth);
    var sendTransaction = web3.eth.sendTransaction.bind(web3.eth);
    web3.eth.sendTransaction = function(options, f) {
        // No callback - use compatibility sync API.
        if (typeof f != "function")
            return sendTransaction(options);
        // Callback - use async API.
        var id = postTransaction(options);
        var timer_id = window.setInterval(check, 500);
        function check() {
            let r = web3.eth.checkTransaction(id);
            if (typeof r == 'string') {
                clearInterval(timer_id);
                if (r == "0x0000000000000000000000000000000000000000000000000000000000000000")
                    f("Rejected", r);
                else
                    f(null, r);
            }
        }
    }
}

[coveralls]

Coverage decreased (-0.09%) to 76.288% when pulling a465d01 on async_sendtx into 5ab18d1 on master.

[coveralls]

Coverage decreased (-0.1%) to 76.297% when pulling 73c9125 on async_sendtx into dd17c76 on master.

[tomusdrw]

It can leak (when check is not called), maybe TransientHashMap (the same one is used for filters) could be used here?

[tomusdrw]

JS code is invalid in general case - The code should be applied only if signer is enabled.

Minor things:

1. Rejected -> { message: 'rejected' } for compatibility with Mist.
2. If postTransaction or checkTransaction fail (for instance when node is turned off or API returns invalid params error) callback is not invoked (you need to try/catch the invocations when using sync methods)

[gavofyork]

signer should always be used with UI and in any case JS code is not part of this PR.

[coveralls]

Coverage decreased (-0.01%) to 76.39% when pulling 5864bcc on async_sendtx into dd17c76 on master.