sync-dd: use timeout(1) instead of rsync's --timeout option

We want to abort the rsync after a fixed time; rsync's --timeout
option only does so if no I/O at all happens for the specified time.
For I/O starved systems this can still mean that the sync takes way
too long.

Also reduce the timeout to 30m (from 60m).  If we miss syncing
something, it will get synced with the next run anyway.