Preparations for removing BaseDendrite (#3016)

Preparations to actually remove/replace `BaseDendrite`.
Quite a few changes:
- SyncAPI accepts an `fulltext.Indexer` interface (fulltext is removed
from `BaseDendrite`)
- Caches are removed from `BaseDendrite`
- Introduces a `Router` struct (likely to change)
  - also fixes #2903
- Introduces a `sqlutil.ConnectionManager`, which should remove
`base.DatabaseConnection` later on
- probably more

mediaapi/mediaapi.go

@@ -32,12 +32,12 @@ func AddPublicRoutes(
 	cfg := &base.Cfg.MediaAPI
 	rateCfg := &base.Cfg.ClientAPI.RateLimiting
 
-	mediaDB, err := storage.NewMediaAPIDatasource(base, &cfg.Database)
+	mediaDB, err := storage.NewMediaAPIDatasource(base.ConnectionManager, &cfg.Database)
 	if err != nil {
 		logrus.WithError(err).Panicf("failed to connect to media db")
 	}
 
 	routing.Setup(
-		base.PublicMediaAPIMux, cfg, rateCfg, mediaDB, userAPI, client,
+		base.Routers.Media, cfg, rateCfg, mediaDB, userAPI, client,
 	)
 }