Add option to control parallelism getting items (#2864)

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.

This PR does not expose this option at the CLI layer

Manually tested to ensure it
* uses the default if options value is 0
* uses the default if options value > default
* uses option value otherwise

There's another PR open for this but it's not expected to main. We still
want to make sure the change goes in though, hence this PR.

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Test
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Test Plan

- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

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 < 1 || 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

@@ -6,12 +6,13 @@ import (
 
 // Options holds the optional configurations for a process
 type Options struct {
-	Collision          CollisionPolicy `json:"-"`
+	Collision            CollisionPolicy `json:"-"`
-	DisableMetrics     bool            `json:"disableMetrics"`
+	DisableMetrics       bool            `json:"disableMetrics"`
-	FailFast           bool            `json:"failFast"`
+	FailFast             bool            `json:"failFast"`
-	RestorePermissions bool            `json:"restorePermissions"`
+	RestorePermissions   bool            `json:"restorePermissions"`
-	SkipReduce         bool            `json:"skipReduce"`
+	SkipReduce           bool            `json:"skipReduce"`
-	ToggleFeatures     Toggles         `json:"ToggleFeatures"`
+	ItemFetchParallelism int             `json:"itemFetchParallelism"`
+	ToggleFeatures       Toggles         `json:"ToggleFeatures"`
 }
 
 // Defaults provides an Options with the default values set.