zerver: Move fts_update_log table id to bigint.

This table, while it does not contain a large number of rows, consumes
a primary key for every message send and update.  Since it is not
referenced by any other table, the migration is simple; and since it
does not contain many rows at any time, it should be fast.

zerver/migrations/0529_fts_bigint_id.py

@@ -0,0 +1,20 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0528_realmauditlog_zerver_realmauditlog_user_activations_idx"),
+    ]
+
+    operations = [
+        migrations.RunSQL(
+            sql="alter table fts_update_log alter column id set data type bigint",
+            reverse_sql="alter table fts_update_log alter column id set data type int",
+            elidable=True,
+        ),
+        migrations.RunSQL(
+            sql="alter sequence fts_update_log_id_seq as bigint",
+            reverse_sql="alter sequence fts_update_log_id_seq as int",
+            elidable=True,
+        ),
+    ]