i2p: use pointers to Sock to accommodate mocking

Change the types of `i2p::Connection::sock` and
`i2p::sam::Session::m_control_sock` from `Sock` to
`std::unique_ptr<Sock>`.

Using pointers would allow us to sneak `FuzzedSock` instead of `Sock`
and have the methods of the former called.

After this change a test only needs to replace `CreateSock()` with
a function that returns `FuzzedSock`.

src/net.cpp

@@ -432,7 +432,7 @@ CNode* CConnman::ConnectNode(CAddress addrConnect, const char *pszDest, bool fCo
             i2p::Connection conn;
             if (m_i2p_sam_session->Connect(addrConnect, conn, proxyConnectionFailed)) {
                 connected = true;
-                sock = std::make_unique<Sock>(std::move(conn.sock));
+                sock = std::move(conn.sock);
                 addr_bind = CAddress{conn.me, NODE_NONE};
             }
         } else if (GetProxy(addrConnect.GetNetwork(), proxy)) {
@@ -2221,7 +2221,7 @@ void CConnman::ThreadI2PAcceptIncoming()
             continue;
         }
 
-        CreateNodeFromAcceptedSocket(conn.sock.Release(), NetPermissionFlags::PF_NONE,
+        CreateNodeFromAcceptedSocket(conn.sock->Release(), NetPermissionFlags::PF_NONE,
                                      CAddress{conn.me, NODE_NONE}, CAddress{conn.peer, NODE_NONE});
     }
 }