Add option to control parallelism getting items

This option is only consumed by Exchange right now and will control the
number of items fetched in parallel per collection. Note that the total
number of parallel item fetches may be higher is kopia is concurrently
uploading multiple collections.

src/internal/connector/exchange/exchange_data_collection.go

@@ -197,7 +197,21 @@ func (col *Collection) streamItems(ctx context.Context, errs *fault.Bus) {
 	}
 
 	// Limit the max number of active requests to GC
-	semaphoreCh := make(chan struct{}, urlPrefetchChannelBufferSize)
+	fetchParallelism := col.ctrl.ItemFetchParallelism
+	if fetchParallelism == 0 || fetchParallelism > urlPrefetchChannelBufferSize {
+		fetchParallelism = urlPrefetchChannelBufferSize
+		logger.Ctx(ctx).Infow(
+			"fetch parallelism value not set or out of bounds, using default",
+			"default_parallelism",
+			urlPrefetchChannelBufferSize,
+			"requested_paralellism",
+			col.ctrl.ItemFetchParallelism,
+		)
+	}
+
+	logger.Ctx(ctx).Infow("fetching data with parallelism", "fetch_parallelism", fetchParallelism)
+
+	semaphoreCh := make(chan struct{}, fetchParallelism)
 	defer close(semaphoreCh)
 
 	// delete all removed items

src/pkg/control/options.go

@@ -11,6 +11,7 @@ type Options struct {
 	FailFast             bool            `json:"failFast"`
 	RestorePermissions   bool            `json:"restorePermissions"`
 	SkipReduce           bool            `json:"skipReduce"`
+	ItemFetchParallelism int             `json:"itemFetchParallelism"`
 	ToggleFeatures       Toggles         `json:"ToggleFeatures"`
 }