Optimize AbstractBlob#equal to not do content equals when possible

AbstractBlob#equals tries to match content when length is equal and content identities is not null and different. Matching content triggers an expensive download of binaries for S3DataStore.
Since, right now the content identity is the content hash the check can be short -circuited when the content identities is not null and not equal to return false.

This can be revisited if we change the identity to something different.