Direct zip-to-zip copy of files when recreating elements
Merging result bundles is a very common action performed by the broker. Currently this decompresses the attachments from one result bundle, and (re)compresses them into another result bundle.
Copying of files in ContentSrcElement._copy should be deferred until Bundle.to_* is executed.
This now makes it possible to make the copying of files smarter by copying compressed data from the source zip file into the destination zip file without recompressing.
As a bonus, this delays blocking system call to Bundle.to_* as well, making Element.(re)create safe to use in an async environment.