extract background downloader to its own class

lbry/stream/background_downloader.py

@@ -0,0 +1,21 @@
+import asyncio
+
+from lbry.stream.downloader import StreamDownloader
+
+
+class BackgroundDownloader:
+    def __init__(self, conf, storage, blob_manager, dht_node):
+        self.storage = storage
+        self.blob_manager = blob_manager
+        self.node = dht_node
+        self.conf = conf
+
+    async def download_blobs(self, sd_hash):
+        downloader = StreamDownloader(asyncio.get_running_loop(), self.conf, self.blob_manager, sd_hash)
+        try:
+            await downloader.start(self.node, save_stream=False)
+        except ValueError:
+            return
+        for blob_info in downloader.descriptor.blobs[:-1]:
+            await downloader.download_stream_blob(blob_info)
+        await self.storage.set_announce(sd_hash, downloader.descriptor.blobs[0].blob_hash)