Extend run_phase using set_drain_time and all_dropped callback after all objection drops

It is more often that two or more components in verification environment are not in sync with each other. And there may be a case where driver/transmitter finish it's job first and call item_done. After item_done is called from driver and if there is no action pending in sequence then start method of sequence finishes and objection is also dropped.
But we want that simulation (more precisely we can say run_phase) should extend to some more time after all objection are dropped, so that other components which are late with respect to driver can finish it's job. We can achieve this by using set_drain_time in UVM.

Let's go through example and see how we can achieve this using set_drain_time in UVM.
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UVM also provides callback hookups when every objection raises and drops and when all objection drops.
Let's go through example and see how it works,

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Once all the objections are dropped, Drain time takes effect. After Drain time is finished, all_dropped callback takes effect.