feat(dot/sync): improve worker pool

The main difference in the worker pool API is that SubmitBatch() does
not block until the whole batch has been processed. Instead, it returns
an ID which can be used to retrieve the current state of the batch.
In addition, Results() returns a channel over which task results are
sent as they become available.

The main improvement this brings is increased concurrency, since results
can be processed before the whole batch has been completed.

What has not changed is the overall flow of the Strategy interface;
getting a new batch of tasks with NextActions() and processing the
results with Process().

Closes #4232

dot/sync/configuration.go

@@ -17,7 +17,7 @@ func WithStrategies(currentStrategy, defaultStrategy Strategy) ServiceConfig {
 func WithNetwork(net Network) ServiceConfig {
 	return func(svc *SyncService) {
 		svc.network = net
-		svc.workerPool = newSyncWorkerPool(net)
+		//svc.workerPool = newSyncWorkerPool(net)
 	}
 }