Populate exchange + support for queries with variables

[andyrichardson]

Looking to explore how we can extract nodes in a Query document which contain variables (in the populate exchange).

Options:

• Ignore nodes with variables
• Auto populate variables with last used values
• Add a decorator to allow the user to choose what variables to include/exclude .etc

CC @imranolas

[andyrichardson]

Going to reopen this as it's still an ongoing issue.

After some discussion, the plan is to remove tracking of nodes which contain variables in the populate exchange. This will prevent issues where @populate returns nodes which have variables which do not exist.