Signature of KeyValue::Atomic::Increment

[karthik2804]

Currently, the signature of the atomic increment is

increment: func(bucket: bucket, key: key, delta: u64) -> result<u64, error>;

I would like to understand the reasoning behind the delta argument and the return u64 types. This would not support cases where atomic decrement is desired. Would it be better for the types to be i64 to allow for flexibility?