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/storage/storage_test.go

@@ -5,6 +5,7 @@ import (
 	"reflect"
 	"testing"
 
+	"github.com/matrix-org/dendrite/internal/sqlutil"
 	"github.com/matrix-org/dendrite/mediaapi/storage"
 	"github.com/matrix-org/dendrite/mediaapi/types"
 	"github.com/matrix-org/dendrite/setup/config"
@@ -13,7 +14,8 @@ import (
 
 func mustCreateDatabase(t *testing.T, dbType test.DBType) (storage.Database, func()) {
 	connStr, close := test.PrepareDBConnectionString(t, dbType)
-	db, err := storage.NewMediaAPIDatasource(nil, &config.DatabaseOptions{
+	cm := sqlutil.NewConnectionManager()
+	db, err := storage.NewMediaAPIDatasource(cm, &config.DatabaseOptions{
 		ConnectionString: config.DataSource(connStr),
 	})
 	if err != nil {