[media] Clear the stream media cache on the first update.

The stream media cache can easily grow in MB in size. Storing it in
Redis means impacting the performance of the application, as on every
web media streaming event it'll have to fetch and deserialize MBs of
data, and Redis may also flush the .rdb file to disk several times in
the process.

platypush/backend/http/app/streaming/plugins/media/_register.py

@@ -3,7 +3,9 @@ from typing import Optional
 from platypush.backend.http.app.utils import logger, send_request
 from platypush.backend.http.media.handlers import MediaHandler
 
-from ._registry import load_media_map, save_media_map
+from ._registry import clear_media_map, load_media_map, save_media_map
+
+_init = False
 
 
 def get_media_url(media_id: str) -> str:
@@ -17,6 +19,12 @@ def register_media(source: str, subtitles: Optional[str] = None) -> MediaHandler
     """
     Registers a media file and returns its associated media handler.
     """
+    global _init
+
+    if not _init:
+        clear_media_map()
+        _init = True
+
     media_id = MediaHandler.get_media_id(source)
     media_url = get_media_url(media_id)
     media_map = load_media_map()

platypush/backend/http/app/streaming/plugins/media/_registry.py

@@ -43,3 +43,12 @@ def save_media_map(new_map: MediaMap):
     with media_map_lock:
         redis = get_redis()
         redis.mset({MEDIA_MAP_VAR: json.dumps(new_map, cls=Message.Encoder)})
+
+
+def clear_media_map():
+    """
+    Clears the media map from the server.
+    """
+    with media_map_lock:
+        redis = get_redis()
+        redis.delete(MEDIA_MAP_VAR)