Improve safety for weed shells `ec.encode`.
The current process for `ec.encode` is:
1. EC shards for a volume are generated and added to a single server
2. The original volume is deleted
3. EC shards get re-balanced across the entire topology
It is then possible to lose data between #2 and #3, if the underlying volume storage/server/rack/DC
happens to fail, for whatever reason. As a fix, this MR reworks `ec.encode` so:
* Newly created EC shards are spread across all locations for the source volume.
* Source volumes are deleted only after EC shards are converted and balanced.
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