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.go

@@ -65,8 +65,7 @@ func authenticateJWT(accessKey, secretKey string, expiry time.Duration) (string,
 
 	// Validate secret key.
 	// Using bcrypt to avoid timing attacks.
-	hashedSecretKey, _ := bcrypt.GenerateFromPassword([]byte(serverCred.SecretKey), bcrypt.DefaultCost)
-	if bcrypt.CompareHashAndPassword(hashedSecretKey, []byte(secretKey)) != nil {
+	if bcrypt.CompareHashAndPassword(serverCred.SecretKeyHash, []byte(secretKey)) != nil {
 		return "", errAuthentication
 	}