Allow waiting for a specified number of confirmations for contract transactions.
This change only requires an optional TransactionResponse return type to be added to the contract function. This allows the transaction hash to be passed to `.wait`.
For example, previously the `mint` method looked like this without a return value:
```
method mint(token: TestToken, holder: Address, amount: UInt256) {.base, contract.}
```
it still works without a return value, but if we want to wait for a 3 confirmations, we can now define it like this:
```
method mint(token: TestToken, holder: Address, amount: UInt256): ?TransactionResponse {.base, contract.}
```
and use like this:
```
let receipt = await token.connect(signer0)
.mint(accounts[1], 100.u256)
.wait(3) # wait for 3 confirmations
```
Allows specified number of transaction confirmations to be awaited before a transaction is considered finalised. Polls for a transaction receipt then listens for new blocks and calculates the number of blocks between the receipt and the new block.
Notes
1. Only works with websockets currently
2. Replaced transaction not supported yet
Ensures that even when a call to the ethereum node to
unsubscribe fails, it will release the memory associated
with the subscription, including the subscription handler
closure.
Eric Mastro
Mark Spanbroek
Michael Bradley, Jr