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.go

@@ -0,0 +1,54 @@
+// Copyright 2023 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sqlutil
+
+import (
+	"database/sql"
+	"fmt"
+
+	"github.com/matrix-org/dendrite/setup/config"
+)
+
+type Connections struct {
+	db     *sql.DB
+	writer Writer
+}
+
+func NewConnectionManager() Connections {
+	return Connections{}
+}
+
+func (c *Connections) Connection(dbProperties *config.DatabaseOptions) (*sql.DB, Writer, error) {
+	writer := NewDummyWriter()
+	if dbProperties.ConnectionString.IsSQLite() {
+		writer = NewExclusiveWriter()
+	}
+	if dbProperties.ConnectionString != "" || c.db == nil {
+		var err error
+		// Open a new database connection using the supplied config.
+		c.db, err = Open(dbProperties, writer)
+		if err != nil {
+			return nil, nil, err
+		}
+		c.writer = writer
+		return c.db, c.writer, nil
+	}
+	if c.db != nil && c.writer != nil {
+		// Ignore the supplied config and return the global pool and
+		// writer.
+		return c.db, c.writer, nil
+	}
+	return nil, nil, fmt.Errorf("no database connections configured")
+}