Unshelving a file from Unreal locks unreal. Killing process leaves the file locked and can't be checked out

Unreal currently locks any assets that are loaded. Actions such as "revert" from within the editor handle this properly by unloading the asset but unshelving from within the editor does not respect this, which leads to the process hanging and leaves the file in a bad state which the user cannot recover from without p4 admin help.

1. Checkout a file in unreal
2. Shelve the file from unreal
3. Revert the file from unreal
4. Unshelve from unreal
5. The "Unshelving File" progress bar will appear and never finish