feat: safer governance implementation initialization

- init staking proxy to address(1) in the constructor to prevent direct calls
- fix virtual modifiers definition as hardhat coverage still not capable of reading it correctly.
- add comment for help/safety for future upgrades

contracts/governance/RigoblockGovernance.sol

@@ -42,9 +42,11 @@ contract RigoblockGovernance is
 
     /// @notice Constructor has no inputs to guarantee same deterministic address.
     /// @dev Different parameters on each would result in different implementation address.
-    /// @dev Setting staking proxy effectively locks direct calls to this contract.
-    constructor(address initializer) MixinImmutables(address(this), initializer) {
+    /// @dev Initializing to address(1) effectively locks direct calls to implementation.
+    constructor(address initializer)
+        MixinImmutables(address(this), initializer)
+        MixinInitializer(address(1))
+    {
         paramsWrapper().treasuryParameters.proposalThreshold = type(uint256).max;
-        stakingProxy().value = address(0);
     }
 }

contracts/governance/mixins/MixinInitializer.sol

@@ -24,7 +24,15 @@ import "./MixinAbstract.sol";
 import "./MixinStorage.sol";
 
 abstract contract MixinInitializer is MixinStorage, MixinAbstract {
-    modifier onlyDelegatecall() virtual;
+    // This modifier is not necessary as implementation initialization is prevented
+    //  by asserting staking proxy null inside the method, but we keep it for future
+    //  upgrades extra security to prevent accidental initialization.
+    modifier onlyDelegatecall() virtual {_;}
+
+    /// @notice We lock future calls to this implementation.
+    constructor(address stakingProxy_) {
+        stakingProxy().value = stakingProxy_;
+    }
 
     /// @inheritdoc IGovernanceInitializer
     function initializeGovernance(

contracts/governance/mixins/MixinUpgrade.sol

@@ -23,7 +23,7 @@ import "../../utils/storageSlot/StorageSlot.sol";
 import "./MixinStorage.sol"; // storage inherits from interface which declares events
 
 abstract contract MixinUpgrade is MixinStorage {
-    modifier onlyDelegatecall() virtual;
+    modifier onlyDelegatecall() virtual {_;}
 
     /// @inheritdoc IGovernanceUpgrade
     function upgradeImplementation(address newImplementation) external onlyDelegatecall override {