allow IAM cache load to be granular and capture missed state (#14930)

anything that is stuck on the disk today can cause latency
spikes for all incoming S3 I/O, we need to have this
de-coupled so that we can make sure that latency in loading
credentials are not reflected back to the S3 API calls.

The approach this PR takes is by checking if the calls were
updated just in case when the IAM load was in progress,
so that we can use merge instead of "replacement" to avoid
missing state.

cmd/admin-handlers-users-race_test.go

@@ -26,6 +26,7 @@ package cmd
 import (
 	"context"
 	"fmt"
+	"runtime"
 	"sync"
 	"testing"
 	"time"
@@ -41,6 +42,10 @@ func runAllIAMConcurrencyTests(suite *TestSuiteIAM, c *check) {
 }
 
 func TestIAMInternalIDPConcurrencyServerSuite(t *testing.T) {
+	if runtime.GOOS == globalWindowsOSName {
+		t.Skip("windows is clunky")
+	}
+
 	baseTestCases := []TestSuiteCommon{
 		// Init and run test on FS backend with signature v4.
 		{serverType: "FS", signer: signerV4},