Implement proxy contract

[karim-en]

Motivation

We need to extend eth-connector to be able to do the following:

• Pause the contract with one click without multisig.
• Restrict deposit for non-legitimate addresses to near after the integration with Aethos.
• Add the possibility to finalize the pre-migration transactions.

Implementation

eth-connector is not upgradeable so direct change of implementation is not possible. However, it extends AdminControlled. According to AdminControlled implementation, the admin has control over the contract's storage and can pause contract functionality for everyone but themselves. Therefore it was decided that the simplest way to extend eth-connector's functionality is to:

• Create a new proxy contract
• Make it admin of eth-connector
• Pause all eth-connector functions for everyone but the admin
• Route new requests through the proxy contract

As a result we can get all 3 of the required upgrades:

• One-click pauseability is easily achieved by implementing it on the proxy contract.
• Same goes for any logic related to restricting deposits.
• To finalize withdrawals for the old proof producer (pre-migration) we use admin's ability to override storage slots on eth-connector. We override it to the old producer before making a withdrawal and then return to the original value at the end of the transaction.

TODO?

• move SelectivePausableUpgradable.sol to the shared repo

[karim-en]

@kiseln Please add a generic method (Dynamic) to call any impl method.
Please also add a test where you change the admin address over it.

[kiseln]

@kiseln Please add a generic method (Dynamic) to call any impl method. Please also add a test where you change the admin address over it.

See ec064b5