use ParseForm() to allow query param lookups once (#12900)

```
cpu: Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz
BenchmarkURLQueryForm
BenchmarkURLQueryForm-4         247099363                4.809 ns/op           0 B/op          0 allocs/op
BenchmarkURLQuery
BenchmarkURLQuery-4              2517624               462.1 ns/op           432 B/op          4 allocs/op
PASS
ok      github.com/minio/minio/cmd      3.848s
```

cmd/url_test.go

@@ -0,0 +1,70 @@
+// Copyright (c) 2015-2021 MinIO, Inc.
+//
+// This file is part of MinIO Object Storage stack
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package cmd
+
+import (
+	"net/http"
+	"testing"
+)
+
+func BenchmarkURLQueryForm(b *testing.B) {
+	req, err := http.NewRequest(http.MethodGet, "http://localhost:9000/bucket/name?uploadId=upload&partNumber=1", nil)
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	// benchmark utility which helps obtain number of allocations and bytes allocated per ops.
+	b.ReportAllocs()
+	// the actual benchmark for PutObject starts here. Reset the benchmark timer.
+	b.ResetTimer()
+
+	if err := req.ParseForm(); err != nil {
+		b.Fatal(err)
+	}
+
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			req.Form.Get("uploadId")
+		}
+	})
+
+	// Benchmark ends here. Stop timer.
+	b.StopTimer()
+}
+
+// BenchmarkURLQuery - benchmark URL memory allocations
+func BenchmarkURLQuery(b *testing.B) {
+	req, err := http.NewRequest(http.MethodGet, "http://localhost:9000/bucket/name?uploadId=upload&partNumber=1", nil)
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	// benchmark utility which helps obtain number of allocations and bytes allocated per ops.
+	b.ReportAllocs()
+	// the actual benchmark for PutObject starts here. Reset the benchmark timer.
+	b.ResetTimer()
+
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			req.URL.Query().Get("uploadId")
+		}
+	})
+
+	// Benchmark ends here. Stop timer.
+	b.StopTimer()
+}