@JsonIgnore
    public Instant getExpiryDate() {
        return Instant.ofEpochMilli(expiryDateEpochMs);
    }
Represents the hashed form of a Data Feed Key, i.e. where we only have the
hash of the key and not the key itself.
stroom-receive/stroom-receive-common/src/main/java/stroom/receive/common/HashedDataFeedKey.java