Use more real-world example in execution_plan_test

For generic coder, triaging issues should be familiar to see what's going on
there.
Refactor execution plan assertion
Sequence dependent actions
Specify deps in input format
References in plan_action
Add reference when input_format mentions subscribed class
Sequences for dependent steps
Multi-deps support
Support for dependencies from array
Extract common test example to separate file
Simple executor implementation
Get Rails integration into separate branch

This reverts commit c0c181d061282d7602b386b87978c5c1a8278104.
Fix failing tests
Restore the references (incomplete)
Persisted steps cache when loading execution plan
Persist run_plan
Move active record persistence into lib subdirectory

Allows using this gem and the active record persistence without rails.
Rename RailsBus to ActiveRecordBus
Migrations path for adding to existing rails app
Dynflow::Bus.using for determining the bus to be used