Use fnv32a hash in the compactor

[pracucci]

In #2887 we learned that fnv32a hash acts better when hashed values slightly change (eg. tenant IDs). We should use it in the blocks compactor sharding as well.

[maxbrodin]

I might be wrong, but doesn't compactor already use fnv32a

// Hash the user ID.
hasher := fnv.New32a()
_, _ = hasher.Write([]byte(userID))
userHash := hasher.Sum32()

[pracucci]

Yeah, my fault. I opened the issue without even looking at the code 🤦