Add execution plan chaining

[adamruzicka]

This commit enables execution plans to be chained. Assuming there is an execution plan EP1, another execution plan EP2 can be chained onto EP1. When chained, EP2 will stay in scheduled state until EP1 goes to stopped state. An execution plan can be chained onto multiple prerequisite execution plans, in which case it will be run once all the prerequisite execution plans are stopped.

It builds on mechanisms which were already present. When an execution plan is chained, it behaves in the same way as if it was scheduled for future execution. A record is created in dynflow_delayed_table and once the conditions for it to execute are right, it is dispatched by the delayed executor. Because of this, there might be small delay between when the prerequisites finishs and the chained plan is started.

TODOs:

• ? somehow show the prerequisite execution plans in the console

[ianballou]

This will be helpful to solve the issue in Katello where a composite content view auto publish can be triggered for content views that are still publishing. They are separate execution chains that need to rely on each other.