GSN compatibility (#1880)

* switch to using Context internally

* add context import

* Add smoke test to make sure enabling GSN support works

* Update test/GSN/ERC721GSNRecipientMock.test.js

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

* Upgrade truffle

* add missing awaits

* Revert "Upgrade truffle"

This reverts commit f9b0ba9019.

contracts/mocks/EscrowMock.sol

@@ -4,6 +4,6 @@ import "../payment/escrow/Escrow.sol";
 
 contract EscrowMock is Escrow {
     constructor() public {
-        Escrow.initialize(msg.sender);
+        Escrow.initialize(_msgSender());
     }
 }