First off, I do know this is an old thread and since there hasn't been a 'real' solution posted, I figured I would update it just in case anyone else comes across this issue as I did. I let my core try to download the mame-metadata for 4 days without touching the system. Only when it locked up my MDs did I take action. After rebooting the core, it would hang on 'Installing: lmce-mame-metadata' and my MDs would not boot - kind of an issue for a MC, so here is the solution I used:
ssh into the core, yes it will let you.
remove the locks on the system from the packages trying to download
sudo rm /var/lib/dpkg/lock
sudo rm /var/cache/apt/archives/lock
and finally install the package manually
sudo apt-get install lmce-mame-metadata
this method will also show the progress, time remaining, etc.