Refactor common code into Croogo::getBulkProcessVars()

Refinement of 67a848251edd3 that's also DRYer
Fix: Don't overwrite nodes.path when it's already set

Closes #475
Fix: load() only insert the first record

Initialize $uniqueKeys for each model. If it's not reset, the isset()
check will cause it to skip since the variable was set in the previous
iteration.