fix: use its own lock in serverConfigV17 (#4014)

Previously serverConfigV17 used a global lock that made any instance of
serverConfigV17 depended on single global serverConfigMu.

This patch fixes by having individual lock per instances.

cmd/config-v17_test.go

@@ -309,7 +309,7 @@ func TestValidateConfig(t *testing.T) {
 		if werr := ioutil.WriteFile(configPath, []byte(testCase.configData), 0700); werr != nil {
 			t.Fatal(werr)
 		}
-		verr := validateConfig()
+		_, verr := getValidConfig()
 		if testCase.shouldPass && verr != nil {
 			t.Errorf("Test %d, should pass but it failed with err = %v", i+1, verr)
 		}