put synctoken into common module

crates/store/src/synctoken.rs

@@ -0,0 +1,13 @@
+const SYNC_NAMESPACE: &str = "github.com/lennart-k/rustical/ns/";
+
+pub fn format_synctoken(synctoken: i64) -> String {
+    format!("{}{}", SYNC_NAMESPACE, synctoken)
+}
+
+pub fn parse_synctoken(synctoken: &str) -> Option<i64> {
+    if !synctoken.starts_with(SYNC_NAMESPACE) {
+        return None;
+    }
+    let (_, synctoken) = synctoken.split_at(SYNC_NAMESPACE.len());
+    synctoken.parse::<i64>().ok()
+}