Apply streaming_connections_per_host to decommission

The streaming_connections_per_host parameter can be used to speed up
CPU-bound bootstrap. Semantically there seems nothing wrong to use it
during decommission as well instead of the hardcoded 1 connection.

src/java/org/apache/cassandra/service/StorageService.java

@@ -5990,7 +5990,7 @@ private Future<StreamState> streamRanges(Map<String, EndpointsByReplica> rangesT
             sessionsToStreamByKeyspace.put(keyspace, replicasPerEndpoint.build());
         }
 
-        StreamPlan streamPlan = new StreamPlan(StreamOperation.DECOMMISSION);
+        StreamPlan streamPlan = new StreamPlan(StreamOperation.DECOMMISSION, DatabaseDescriptor.getStreamingConnectionsPerHost());
 
         // Vinculate StreamStateStore to current StreamPlan to update transferred rangeas per StreamSession
         streamPlan.listeners(streamStateStore);

src/java/org/apache/cassandra/streaming/StreamPlan.java

@@ -56,6 +56,11 @@ public StreamPlan(StreamOperation streamOperation)
         this(streamOperation, 1, false, NO_PENDING_REPAIR, PreviewKind.NONE);
     }
 
+    public StreamPlan(StreamOperation streamOperation, int connectionsPerHost)
+    {
+        this(streamOperation, connectionsPerHost, false, NO_PENDING_REPAIR, PreviewKind.NONE);
+    }
+
     public StreamPlan(StreamOperation streamOperation, boolean connectSequentially)
     {
         this(streamOperation, 1, connectSequentially, NO_PENDING_REPAIR, PreviewKind.NONE);