Improve EC shards rebalancing logic across racks.
- Favor target shards with less preexisting shards, to ensure a fair distribution.
- Randomize selection when multiple possible target shards are available.
- Add logic to account for replication settings when selecting target shards (currently disabled).
* Rename `command_ec_encode_test.go` to `command_ec_common_test.go`.
All tests defined in this file are now for `command_ec_common.go`.
* Minor code cleanups.
- Fix broken `ec.balance` test.
- Rework integer ceiling division to not use floats, which can introduce precision errors.
* Introduce logic to resolve volume replica placement within EC rebalancing.
This will be used to make rebalancing logic topology-aware.
* Give shell.EcNode.dc a dedicated DataCenterId type.
