Constant akd_client::TOMBSTONE

pub const TOMBSTONE: &'static [u8];

A “tombstone” is a false value in an AKD ValueState denoting that a real value has been removed (e.g. data rentention policies). Should a tombstone be encountered, we have to assume that the hash of the value is correct, and we move forward without being able to verify the raw value. We utilize an empty array to save space in the storage layer

See GitHub issue #130 for more context