jwt: Cache the bcrypt password hash. (#3526)

Creds don't require secretKeyHash to be calculated
everytime, cache it instead and re-use.

This is an optimization for bcrypt.

Relevant results from the benchmark done locally, negative
value means improvement in this scenario.

```
benchmark                       old ns/op     new ns/op     delta
BenchmarkAuthenticateNode-4     160590992     80125647      -50.11%
BenchmarkAuthenticateWeb-4      160556692     80432144      -49.90%

benchmark                       old allocs     new allocs     delta
BenchmarkAuthenticateNode-4     87             75             -13.79%
BenchmarkAuthenticateWeb-4      87             75             -13.79%

benchmark                       old bytes     new bytes     delta
BenchmarkAuthenticateNode-4     15222         9785          -35.72%
BenchmarkAuthenticateWeb-4      15222         9785          -35.72%
```

cmd/jwt_test.go

@@ -75,10 +75,40 @@ func testAuthenticate(authType string, t *testing.T) {
 	}
 }
 
-func TestNodeAuthenticate(t *testing.T) {
+func TestAuthenticateNode(t *testing.T) {
 	testAuthenticate("node", t)
 }
 
-func TestWebAuthenticate(t *testing.T) {
+func TestAuthenticateWeb(t *testing.T) {
 	testAuthenticate("web", t)
 }
+
+func BenchmarkAuthenticateNode(b *testing.B) {
+	testPath, err := newTestConfig(globalMinioDefaultRegion)
+	if err != nil {
+		b.Fatalf("unable initialize config file, %s", err)
+	}
+	defer removeAll(testPath)
+
+	creds := serverConfig.GetCredential()
+	b.ResetTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		authenticateNode(creds.AccessKey, creds.SecretKey)
+	}
+}
+
+func BenchmarkAuthenticateWeb(b *testing.B) {
+	testPath, err := newTestConfig(globalMinioDefaultRegion)
+	if err != nil {
+		b.Fatalf("unable initialize config file, %s", err)
+	}
+	defer removeAll(testPath)
+
+	creds := serverConfig.GetCredential()
+	b.ResetTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		authenticateWeb(creds.AccessKey, creds.SecretKey)
+	}
+}