Change run_at default to clock_timestamp()

## Problem

Jobs enqueued in transactions can appear delayed as the default `run_at` is `now()`, which is a delayed time in a long transaction.

## Example

We know that the `que_jobs` table has this column: `run_at timestamp with time zone DEFAULT now() NOT NULL,`

We have some jobs that perform work, and enqueue another job. eg:

```ruby
class CalculateBilling < Que::Job
  def run(ref)
    ActiveRecord::Base.transaction do
      calculate_stuff(ref) # Takes 30 seconds
      SendBillingSms.enqueue(ref)
    end
  end
end
```

We then also have an alarm to check our jobs are running in a timely fashion by comparing when the job started with its `run_at`.

In the scenario above, this means the `SendBillingSms` job is enqueued with `run_at` as `now()`. But, `now()` does not mean "the time on the wall" in postgres. It means "the time the transaction started".

See: https://newcubator.com/blog/web/postgresql-timestamp-methods

_Both now() and clock_timestamp() are PostgreSQL functions. The main difference between them is that now() adds the time at the start of the transactions, while clock_timestamp() inserts the time at the execution of the statement._

This means all of our `SendBillingSms` jobs are instantaneously "30 seconds delayed", even if they are run the split second that they are visible to workers.

## Solution

We could/should change the column to have a default of `clock_timestamp()`.

ie: `run_at timestamp with time zone DEFAULT clock_timestamp() NOT NULL,`

This means that the intent of what the `run_at` of a job "to run now" should be is more accurate (probably, ie principle of least surprise). It also means jobs can be monitored for "delayed start" more easily and accurately.

Note, we do not want to use Ruby time in a `run_at` as it may be out, even by a second or two. We also would not like to pass a `run_at` in since:

1. We'd have to do it everywhere, or monkeypatch que
2. We'd have to do a DB lookup for what that time is, which is an extra call

lib/que/migrations.rb

@@ -4,7 +4,7 @@ module Que
   module Migrations
     # In order to ship a schema change, add the relevant up and down sql files
     # to the migrations directory, and bump the version here.
-    CURRENT_VERSION = 7
+    CURRENT_VERSION = 8
 
     class << self
       def migrate!(version:)

lib/que/migrations/8/down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE que_jobs
+ALTER COLUMN run_at SET DEFAULT now();

lib/que/migrations/8/up.sql

@@ -0,0 +1,2 @@
+ALTER TABLE que_jobs
+ALTER COLUMN run_at SET DEFAULT clock_timestamp();

spec/que/migrations.current_schema_spec.rb

@@ -115,6 +115,19 @@ def assert_constraint_error(name, &block)
     end
   end
 
+  describe "que_jobs table column defaults" do
+    it "should make sure that the default of run_at is clock_timestamp()" do
+      results = Que.execute (<<~SQL)
+        SELECT column_default
+        FROM information_schema.columns
+        WHERE table_name = 'que_jobs' AND column_name = 'run_at';
+      SQL
+      column_default = results.to_a.first.fetch(:column_default)
+
+      assert_equal column_default, "clock_timestamp()"
+    end
+  end
+
   describe "que_lockers table constraints" do
     let :locker_attrs do
       {

spec/que/migrations.state_trigger_spec.rb

@@ -43,7 +43,7 @@ def assert_notification(
 
     DB.transaction do
       result = yield
-      current_time = DB.get{now.function}
+      current_time = DB.get{clock_timestamp.function}
 
       id     ||= result
       run_at ||= current_time