DCOM-287: [GH-369] [RFC] Load parent classes when loading metadata from cache

Jira issue originally created by user @doctrinebot:

This issue is created automatically through a Github pull request on behalf of gonzalovilaseca:

Url: https://github.com/doctrine/common/pull/369

Message:

This can be done more neatly, but before putting more time in it I'd like to check if it's a good idea.
I came across this solution because we had a problem with Gedmo extensions when caching metadata in Redis, this is our scenario:

```
Class A extends B;
```

`Class B` has a `Gedmo extension` mapped field.

On first request metadata for both `Class A` and `Class B` is cached in Redis.

On second request `AbstractClassMetadataFactory` loads `Class A` from Redis.

Gedmo extension loops through all `Class A` parents and calls method `hasMetadataFor($parentClass)` in `AbstractClassMetadataFactory`, only `Class A` metadata has been loaded previously, therefore `hasMetadataFor($parentClass)` returns false, and `Gedmo` then skips the parent class.

Should Gedmo exension load the metadata for the parent class or should it be done as proposed in this PR?