perf(db): Use statement trigger for marking deleted sessions

When sessions are deleted, a trigger is used to insert records into the
session_deleted table. This table is utilized by the sessions list
endpoint when using a refresh token to inform a client of the sessions
that have been deleted since the last request. We delete sessions in
bulk via a controller job to delete sessions that have been terminated
over an hour ago, which results in the trigger running a large number of
separate insert statements while processing the delete statement.

This changes the trigger to run once for the delete statement, instead
of for each row, resulting in a single bulk insert statement to the
session_deleted table.

This new trigger function also avoids the use of `on conflict`. When
testing this function, while the single statement was still faster than
running multiple inserts, the `on conflict` still added significant
overhead, even when there were no conflicts. It should be safe to
perform the insert without the `on conflict`, since the same ID should
never be deleted more than once if it is successfully deleted.

internal/db/schema/migrations/oss/postgres/91/04_insert_session_delete_on_statement.up.sql

@@ -0,0 +1,25 @@
+-- Copyright (c) HashiCorp, Inc.
+-- SPDX-License-Identifier: BUSL-1.1
+
+begin;
+  create function bulk_insert_deleted_ids() returns trigger
+  as $$
+  begin
+    execute format('insert into %I (public_id, delete_time)
+                         select o.public_id, now()
+                           from old_table o;',
+                   tg_argv[0]);
+    return null;
+  end;
+  $$ language plpgsql;
+  comment on function bulk_insert_deleted_ids is
+    'bulk_insert_deleted_ids is a function that inserts records into the table '
+    'specified by the first trigger argument. It takes the public IDs from the '
+    'set of rows that where deleted and the current timestamp.';
+
+  drop trigger insert_deleted_id on session;
+  create trigger bulk_insert_deleted_ids
+    after delete on session
+    referencing old table as old_table
+    for each statement execute function bulk_insert_deleted_ids('session_deleted');
+commit;