Add deposit limit to UserDeposit

[pirapira]

This issue keeps track of adding a deposit limit to the UserDeposit. Until some concrete number is decided (https://github.com/raiden-network/team/issues/333), we can set up an arbitrary number (something like 12345678 looks temporary.)

[pirapira]

• the limit should be configurable at the deployment time.

[pirapira]

I have to choose how to keep track of the total balance of UserDeposit.

Options

Token balance of the contract when token.balanceOf(address(this)) is too big, new deposits are declined.

A storage variable a new storage variable whole_balance will be increased and decreased in deposit() and withdraw functions. When whole_balance is too big, new deposits are declined.

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Comparison

• Token balance of the contract can be manipulated by an attacker. An attacker can always send some amount of tokens to the contract, and stop all deposits.
• A storage variable requires some careful coding.
  • but the amount of careful coding is small.

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Conclusion

I'll maintain a storage variable.