qa: background rsync task for FS workunits
A client that just sits there trying to rsync the contents of the filesystem. Running this at the same time as any other workunit should enable us to get some basic coverage of multi-client access to the same files.
Of course, it's also a very realistic workload! This is a stress test rather than a full correctness test, as when we rsync from a filesystem being modified we don't have a straightforward way to verify that what got copied is right.
For bonus points, do a version of this that does a "snapshot, rsync from snapshot, delete snapshot" cycle, which is also a super-realistic workload that would exercise multi-client and snapshots.