Move all IAM storage functionality into iam store type (#13567)

This reverts commit 091a7ae359.

- Ensure all actions accessing storage lock properly.

- Behavior change: policies can be deleted only when they
  are not associated with any active credentials.

Also adds fix for accidental canned policy removal that was present in the
reverted version of the change.

cmd/iam-dummy-store.go

@@ -27,22 +27,37 @@ import (
 
 type iamDummyStore struct {
 	sync.RWMutex
+	*iamCache
+	usersSysType UsersSysType
 }
 
-func (ids *iamDummyStore) lock() {
+func newIAMDummyStore(usersSysType UsersSysType) *iamDummyStore {
+	return &iamDummyStore{
+		iamCache:     newIamCache(),
+		usersSysType: usersSysType,
+	}
+}
+
+func (ids *iamDummyStore) rlock() *iamCache {
+	ids.RLock()
+	return ids.iamCache
+}
+
+func (ids *iamDummyStore) runlock() {
+	ids.RUnlock()
+}
+
+func (ids *iamDummyStore) lock() *iamCache {
 	ids.Lock()
+	return ids.iamCache
 }
 
 func (ids *iamDummyStore) unlock() {
 	ids.Unlock()
 }
 
-func (ids *iamDummyStore) rlock() {
-	ids.RLock()
-}
-
-func (ids *iamDummyStore) runlock() {
-	ids.RUnlock()
+func (ids *iamDummyStore) getUsersSysType() UsersSysType {
+	return ids.usersSysType
 }
 
 func (ids *iamDummyStore) migrateBackendFormat(context.Context) error {