Improved Feature-List Merge Support 

A few new features:
* Use the `genome feature-list import` command underneath, so we get the same validation logic as new imports.
* Try to deduce the correct reference to use in more situations.  (This relies on the connective metadata between references being set up correctly.)

A few fixes:
* Less broken list caching.
* Use the validation subroutine that wasn't being called.
This could be on `Genome::Model::Build::ReferenceSequence`?