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

internal/sqlutil/connection_manager_test.go

@@ -0,0 +1,56 @@
+package sqlutil_test
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/matrix-org/dendrite/internal/sqlutil"
+	"github.com/matrix-org/dendrite/setup/config"
+	"github.com/matrix-org/dendrite/test"
+)
+
+func TestConnectionManager(t *testing.T) {
+	test.WithAllDatabases(t, func(t *testing.T, dbType test.DBType) {
+		conStr, close := test.PrepareDBConnectionString(t, dbType)
+		t.Cleanup(close)
+		cm := sqlutil.NewConnectionManager()
+
+		dbProps := &config.DatabaseOptions{ConnectionString: config.DataSource(string(conStr))}
+		db, writer, err := cm.Connection(dbProps)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		switch dbType {
+		case test.DBTypeSQLite:
+			_, ok := writer.(*sqlutil.ExclusiveWriter)
+			if !ok {
+				t.Fatalf("expected exclusive writer")
+			}
+		case test.DBTypePostgres:
+			_, ok := writer.(*sqlutil.DummyWriter)
+			if !ok {
+				t.Fatalf("expected dummy writer")
+			}
+		}
+
+		// test global db pool
+		dbGlobal, writerGlobal, err := cm.Connection(&config.DatabaseOptions{})
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !reflect.DeepEqual(db, dbGlobal) {
+			t.Fatalf("expected database connection to be reused")
+		}
+		if !reflect.DeepEqual(writer, writerGlobal) {
+			t.Fatalf("expected database writer to be reused")
+		}
+
+		// test invalid connection string configured
+		cm = sqlutil.NewConnectionManager()
+		_, _, err = cm.Connection(&config.DatabaseOptions{ConnectionString: "http://"})
+		if err == nil {
+			t.Fatal("expected an error but got none")
+		}
+	})
+}