refactor: VcardContact: Change timestamp type to i64

- u64 only adds unnecessary conversions.
- `Contact::last_seen` is also `i64`, so make timestamps such everywhere.

deltachat-jsonrpc/src/api/types/contact.rs

@@ -103,7 +103,7 @@ pub struct VcardContact {
     /// Contact color as hex string.
     color: String,
     /// Last update timestamp.
-    timestamp: Option<u64>,
+    timestamp: Option<i64>,
 }
 
 impl From<deltachat_contact_tools::VcardContact> for VcardContact {