MergeUploads optimization

User story

Running the MergeUploads task takes longer than expected for an (apparently) simple combination of 2 .changes files.

Error handling

This is due to relying on mergechanges, a script which comes from devscripts, a large set of Debian tools with has a lot of dependencies; and installing+running it inside a full container (through unshare).

Implementation plan

We could reimplement it directly in Python: !1117 (comment 527761)

and also moving this to a ServerTask (without Worker).

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information