Vault contract for ETH app.

[alistair-singh]

Added a vault to store ether on behalf of EthApp.

• The vault is secured with Ownable openzeppelin contract.
• Only one version of EthApp can hold ownership of the vault at a time.
• EthApp controls transfer of vault ownership via messages from the inbound channel.
• Ownership transfer is assumed to be part of a consistent upgrade procedure. i.e. No new transfers across the bridge can be initiated and all pending transfers are complete.