Sidekiq has a useful optimisation called `push_bulk` that enqueues many jobs at
once, eliminating the repeated Redis roundtrips. However, this feature is not
exposed through Active Job, so it only works for `Sidekiq::Worker` jobs. This
adds a barrier to Active Job adoption for apps that rely on this feature. It
also makes it harder for other queue adapters to implement similar
functionality, as they then have to take care of serialization, callbacks, etc.
themselves.

This commit adds `ActiveJob.perform_all_later(<job1>, <job2>)`, backed by
Sidekiq's `push_bulk` and with a fallback to enqueuing serially if the queue
adapter does not support bulk enqueue.

The performance benefit for 1000 jobs can be more than an order of magnitude:

| Enqueue type       | Serial time (ms) | Bulk time (ms) | Speedup |
| ------------------ | ---------------- | -------------- | ------- |
| Raw Sidekiq        |             2661 |            119 |     22x |
| Active Job Sidekiq |             2853 |            208 |     14x |

(Measured in a simple test app in our production environment.)

Instrumentation for perform_all_later uses a new event `enqueue_all.active_job`