contractcourt: fix concurrent access to resolved

This commit makes `resolved` an atomic bool to avoid data race. This
field is now defined in `contractResolverKit` to avoid code duplication.

contractcourt/htlc_timeout_resolver_test.go

@@ -532,7 +532,7 @@ func testHtlcTimeoutResolver(t *testing.T, testCase htlcTimeoutTestCase) {
 	wg.Wait()
 
 	// Finally, the resolver should be marked as resolved.
-	if !resolver.resolved {
+	if !resolver.resolved.Load() {
 		t.Fatalf("resolver should be marked as resolved")
 	}
 }