Consolidate sort's candidateLimit and Director's serverResLimit

As implemented, the adaptive sort protocol would only return `candidateLimit = 3`
servers for the Director to include in its redirect response. However, we already
have `serverResLimit = 6`, which configures the max number of servers with which
to supply a client request. Since these two variables have the same effect (i.e.
limiting the number of sorted servers sent to clients by the Director), why not
make them the same variable?

That's what this commit does :)

director/sort.go

@@ -68,7 +68,6 @@ const (
 	objAvailabilityFactor    = 2    // Multiplier for knowing whether an object is present
 	loadHalvingThreshold     = 10.0 // Threshold where the load havling factor kicks in
 	loadHalvingFactor        = 4.0  // Halving interval for load
-	candidateLimit           = 3    // Number of servers return
 )
 
 func (me SwapMaps) Len() int {
@@ -253,15 +252,15 @@ func sortServerAds(clientAddr netip.Addr, ads []server_structs.ServerAd, availab
 		case server_structs.RandomType:
 			weights[idx] = SwapMap{rand.Float64(), idx}
 		default:
-			return nil, errors.Errorf("Invalid sort method '%s' set in Director.CacheSortMethod. Valid methods are 'distance',"+
-				"'distanceAndLoad', 'adaptive', and 'random.'", param.Director_CacheSortMethod.GetString())
+			// Never say never, but this should never get hit because we validate the value on startup.
+			return nil, errors.Errorf("Invalid sort method '%s' set in Director.CacheSortMethod.", param.Director_CacheSortMethod.GetString())
 		}
 	}
 
 	if sortMethod == string(server_structs.AdaptiveType) {
-		candidates, _ := stochasticSort(weights, candidateLimit)
+		candidates, _ := stochasticSort(weights, serverResLimit)
 		resultAds := []server_structs.ServerAd{}
-		for _, cidx := range candidates[:candidateLimit] {
+		for _, cidx := range candidates[:serverResLimit] {
 			resultAds = append(resultAds, ads[cidx])
 		}
 		return resultAds, nil