Command Engine: Move indexing and other actions into "onSuccess" method

[ferrys]

As a developer, when calling a command such as CreateDatasetCommand, I want to be able to do things like index the dataset after the dataset as been successfully created and the EJB transaction has completed, so that I will never have a scenario where my dataset was indexed but it does not exist in the database.

@scolapasta and I started a little bit of work related to this issue but determined it needed a bigger discussion surrounding its implementation. The current work on branch 4425-on-success is not what we think the correct solution will be, but just initial work.

[scolapasta]

Two challenges we'll need to solve:
• how to handle nested commands and calling nested onSuccess calls
• if operations in onSuccess fail, how to automate them trying again and/or inform installation admins so it can rerun manually