Don't store min+max namespace IDs in badger inverted index keys

[musalbas]

This should save around 50% of the storage used by the inverted index. We don't need to put min+max namespace IDs in the inverted index, as they don't contribute to collision resistance. To access a key, you can simply truncate the namespace ID.

[moldis]

Is this issue still valid? I dont see any occurrences in the code

[Wondertan]

Yes it is. This happens in a subtle way. The good thing is that soon the inverted index will be removed altogether with this issue via new protocol: #2971