Add `replacing` property to events

I suggest adding replacing property to events. This would make users’ lives easier and safer.

There are two issues with the replacing and replacedBy properties:

1. They aren’t included (both are null) when you query specific transactions, e.g. HTTP GET v1/cryptos/{cryptoCode}/derivations/{derivationScheme}/transactions/{txId}. If you have lots and lots of transactions for your derivation scheme, querying all transactions when you’re only interested in one becomes really impractical.
2. They aren’t included in newtransaction events (I guess replacedBy isn’t applicable, but repacing certainly is)

For me, fixing issue 2 would be preferable, but if that’s not possible, fixing 1 works too. Of course both would be great.

[NicolasDorier]

@kallerosenbaum I have another trouble: replacing should be an array rather than a single value. As a single transaction can replace sevaral other transaction.

[kallerosenbaum]

I have reviewed now, only one question, on #413 . Thanks so much for your efforts!!!