remove unnecessary LRU for internode auth token (#20119)

removes contentious usage of mutexes in LRU, which
were never really reused in any manner; we do not
need it.

To trust hosts, the correct way is TLS certs; this PR completely
removes this dependency, which has never been useful.

```
0  0%  100%  25.83s 26.76%  github.com/hashicorp/golang-lru/v2/expirable.(*LRU[...])
0  0%  100%  28.03s 29.04%  github.com/hashicorp/golang-lru/v2/expirable.(*LRU[...])
```

Bonus: use `x-minio-time` as a nanosecond to avoid unnecessary
parsing logic of time strings instead of using a more
straightforward mechanism.

cmd/lock-rest-server-common_test.go

@@ -44,7 +44,7 @@ func createLockTestServer(ctx context.Context, t *testing.T) (string, *lockRESTS
 		},
 	}
 	creds := globalActiveCred
-	token, err := authenticateNode(creds.AccessKey, creds.SecretKey, "")
+	token, err := authenticateNode(creds.AccessKey, creds.SecretKey)
 	if err != nil {
 		t.Fatal(err)
 	}