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/admin-handlers.go

@@ -131,8 +131,8 @@ func (adminAPI adminAPIHandlers) ServiceCredentialsHandler(w http.ResponseWriter
 	}
 
 	// Avoid setting new credentials when they are already passed
-	// by the environnement
-	if globalEnvAccessKey != "" || globalEnvSecretKey != "" {
+	// by the environment.
+	if globalIsEnvCreds {
 		writeErrorResponse(w, ErrMethodNotAllowed, r.URL)
 		return
 	}