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.

go.mod

@@ -50,7 +50,7 @@ require (
 	github.com/minio/madmin-go v1.1.11-0.20211102182201-e51fd3d6b104
 	github.com/minio/minio-go/v7 v7.0.15
 	github.com/minio/parquet-go v1.0.0
-	github.com/minio/pkg v1.1.5
+	github.com/minio/pkg v1.1.6-0.20211103212545-951bbd71498c
 	github.com/minio/selfupdate v0.3.1
 	github.com/minio/sha256-simd v1.0.0
 	github.com/minio/simdjson-go v0.2.1