Moving updated-at timestamps to unique millisecond times (#17489)

To allow the ability to have guaranteed-unique cursors, we make two
important updates:
* Add new updated_at_ms and created_at_ms columns that are in
millisecond precision
* Guarantee uniqueness -- if multiple items are inserted at the same
millisecond, bump the new one by one millisecond until it becomes unique

This lets us use single-number cursors for forwards and backwards paging
through resultsets and guarantee that the cursor is a fixed point to do
(timestamp > cursor) and get new items only.

This updated implementation is backwards-compatible since multiple
appservers can be running and won't handle the previous method well.

codex-rs/rollout/src/state_db.rs

@@ -5,7 +5,6 @@ use crate::list::ThreadSortKey;
 use crate::metadata;
 use chrono::DateTime;
 use chrono::NaiveDateTime;
-use chrono::Timelike;
 use chrono::Utc;
 use codex_protocol::ThreadId;
 use codex_protocol::dynamic_tools::DynamicToolSpec;
@@ -131,8 +130,7 @@ fn cursor_to_anchor(cursor: Option<&Cursor>) -> Option<codex_state::Anchor> {
         dt.with_timezone(&Utc)
     } else {
         return None;
-    }
-    .with_nanosecond(0)?;
+    };
     Some(codex_state::Anchor { ts, id })
 }