add contract upgradeability

[johannbarbie]

we want to be able to upgrade the contract with protocol changes of the Plasma chain. But the community should have the ability to veto each upgrade. Process envisioned like this:

1. new contract code is deployed and registered for upgrade.
2. timer runs down over x days.
3. if no majority of token holders veto'ed the upgrade, new contract code is enabled.

[eshavkun]

Contracts are now upgradable with ZeppelinOS
New branch pushed: https://github.com/leapdao/leap-contracts/tree/UpgradableContracts

Summary of changes

• Contracts are now Initializable (see /contracts/Initializable.sol)
• Constructors changed to initializers
• Initial values for variables are moved into initializers
• Imports changed from 'openzeppelin-solidity' to 'openzeppelin-eth'
• zos.json created with LeapBridge and ExitToken added
• Basic test: deployed on local (ganache) network and called functions from truffle console

TODO

• Update test suite
• Add upgradability governance - TBD
• Add mock contracts to zos.json - TBD
• Add uint256[50] private ______gap; to the end of contracts that may be inherited from - TBD
• Link to openzeppelin-eth as EVM package - TBD
• Check if libraries and Interfaces - TBD

Deployment

1. Precondition: ZeppelinOS is globally installed (npm install --global zos)
2. zos push --network NETWORK_NAME
3. zos create CONTRACT_NAME --network NETWORK_NAME --init --args "CONSTRUCTOR_PARAMETERS"

Important Notes

• Constructors are not working with ZeppelinOS. Initializers (function initialize() initializer public) are used instead. Initializers are never called automatically (e.g. on contract creation).
• Deployer of the contract becomes its admin. Admin can only call proxy functions (e.g. update contract) but not logic functions. Therefore logic functions must be called from other (non-admin) account

[eshavkun]

Regarding contract upgradability governance:
https://blog.zeppelinos.org/exploring-upgradeability-governance-in-zeppelinos-with-a-gnosis-multisig/

[eezcjkr]

Done #81