convert PullPayment to initializers

contracts/mocks/PullPaymentMock.sol

 pragma solidity ^0.4.24;
 
 
+import "../Initializable.sol";
 import "../payment/PullPayment.sol";
 
 
 // mock class using PullPayment
-contract PullPaymentMock is PullPayment {
+contract PullPaymentMock is Initializable, PullPayment {
 
-  constructor() public payable { }
+  constructor() public payable {
+    PullPayment.initialize();
+  }
 
   // test helper function to call asyncTransfer
   function callTransfer(address dest, uint256 amount) public {

contracts/payment/PullPayment.sol

 pragma solidity ^0.4.24;
 
+import "../Initializable.sol";
 import "./Escrow.sol";
 
 
@@ -8,11 +9,15 @@ import "./Escrow.sol";
  * @dev Base contract supporting async send for pull payments. Inherit from this
  * contract and use _asyncTransfer instead of send or transfer.
  */
-contract PullPayment {
+contract PullPayment is Initializable {
   Escrow private _escrow;
 
-  constructor() public {
+  function initialize() public initializer {
+    // conditional added to make initializer idempotent in case of diamond inheritance
+    if (address(_escrow) == address(0)) {
       _escrow = new Escrow();
+      _escrow.initialize();
+    }
   }
 
   /**