SWORD: use AsyncManager in background tasks
SWORD: use AsyncManager in background tasks

Our implementation of SWORD uses processes to download contents asynchronously.
We do this via the `multiprocessing` package which stopped working when we
moved to gevent. This commit replaces the use of `multiprocessing` with our own
`AsyncManager` which uses threads instead.

This is connected to #312.

artefactual/archivematica-storage-service,artefactual/archivematica-storage-service,artefactual/archivematica-storage-service,artefactual/archivematica-storage-service