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_encode_test.go

@@ -1,31 +0,0 @@
-package shell
-
-import (
-	"fmt"
-	"github.com/seaweedfs/seaweedfs/weed/storage/erasure_coding"
-	"testing"
-)
-
-func TestEcDistribution(t *testing.T) {
-
-	topologyInfo := parseOutput(topoData)
-
-	// find out all volume servers with one slot left.
-	ecNodes, totalFreeEcSlots := collectEcVolumeServersByDc(topologyInfo, "")
-
-	sortEcNodesByFreeslotsDescending(ecNodes)
-
-	if totalFreeEcSlots < erasure_coding.TotalShardsCount {
-		println("not enough free ec shard slots", totalFreeEcSlots)
-	}
-	allocatedDataNodes := ecNodes
-	if len(allocatedDataNodes) > erasure_coding.TotalShardsCount {
-		allocatedDataNodes = allocatedDataNodes[:erasure_coding.TotalShardsCount]
-	}
-
-	for _, dn := range allocatedDataNodes {
-		// fmt.Printf("info %+v %+v\n", dn.info, dn)
-		fmt.Printf("=> %+v %+v\n", dn.info.Id, dn.freeEcSlot)
-	}
-
-}