Avoid removal of some images from cache

**Glide Version**:

**Integration libraries**:4.0.0-RC1

**Device/Android Version**: Independent

**Issue details / Repro steps / Use case background**: I want few images which should not be removed from cache. #501 I followed this to extend glideUrl and sending my specific key but any idea on how to avoid deletion of some specific images from cache?

**Glide load line / `GlideModule` (if any) / list Adapter code (if any)**:
```java
Glide.with...
```

**Layout XML**:
```xml