feat: add index on events for faster revision id (#6170)

On all pods and instances, we run the same revision update query every
second. It is relatively fast when the application has started. This is
the single most ran query in unleash.
Benchmarks:

1. Running pod with existing revisionID:
  - old 5.5ms
  - new 0.028ms

2. New pod without existing revisionID
 - old 9.329ms
 - new 0.033ms



This query is getting optimized


7e66a79f9f/src/lib/features/events/event-store.ts (L161)

src/migrations/20240208130439-events-revision-id-index.js

@@ -0,0 +1,19 @@
+'use strict';
+
+exports.up = function(db, cb) {
+    db.runSql(
+        `
+            CREATE INDEX IF NOT EXISTS idx_events_feature_type_id ON events (id)
+                WHERE feature_name IS NOT NULL
+                    OR type IN ('segment-updated', 'feature_import', 'features-imported');
+
+        `,
+        cb,
+    );
+};
+
+exports.down = function(db, cb) {
+    db.runSql(`
+        DROP INDEX IF EXISTS idx_events_feature_type_id;
+    `, cb);
+};