Fix unstake() error when defer transaction is failed.

[lychees]

You can refer to
https://github.com/Dapp-rosetta/dapp-rosetta/tree/master/Contract/EOS/payout
for detailed.

1. Add singleton_refunds table.
2. Add refund() function.
3. Modify unstake() logic to use refund() function.

Be careful that this repo is using a deprecated version of eosiocpp.
We should first fix this issue then update the version.