Introduce logic to resolve volume replica placement within EC rebalancing. (#6254)

* 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.

weed/shell/command_ec_test.go

@@ -129,7 +129,7 @@ func newEcNode(dc string, rack string, dataNodeId string, freeEcSlot int) *EcNod
 			Id:        dataNodeId,
 			DiskInfos: make(map[string]*master_pb.DiskInfo),
 		},
-		dc:         dc,
+		dc:         DataCenterId(dc),
 		rack:       RackId(rack),
 		freeEcSlot: freeEcSlot,
 	}