Add --id argument to safekeeper setting its unique u64 id.

In preparation for storage node messaging. IDs are supposed to be monotonically
assigned by the console. In tests it is issued by ZenithEnv; at the zenith cli
level and fixtures, string name is completely replaced by integer id. Example
TOML configs are adjusted accordingly.

Sequential ids are chosen over Zid mainly because they are compact and easy to
type/remember.

zenith_utils/src/zid.rs

@@ -221,6 +221,18 @@ impl fmt::Display for ZTenantTimelineId {
     }
 }
 
+// Unique ID of a storage node (safekeeper or pageserver). Supposed to be issued
+// by the console.
+#[derive(Clone, Copy, Eq, Ord, PartialEq, PartialOrd, Debug, Serialize, Deserialize)]
+#[serde(transparent)]
+pub struct ZNodeId(pub u64);
+
+impl fmt::Display for ZNodeId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::fmt::Display;