Transaction writer changes, move roomserver writers (#1285)

* Updated TransactionWriters, moved locks in roomserver, various other tweaks

* Fix redaction deadlocks

* Fix lint issue

* Rename SQLiteTransactionWriter to ExclusiveTransactionWriter

* Fix us not sending transactions through in latest events updater

internal/sqlutil/writer_dummy.go

@@ -0,0 +1,22 @@
+package sqlutil
+
+import (
+	"database/sql"
+)
+
+type DummyTransactionWriter struct {
+}
+
+func NewDummyTransactionWriter() TransactionWriter {
+	return &DummyTransactionWriter{}
+}
+
+func (w *DummyTransactionWriter) Do(db *sql.DB, txn *sql.Tx, f func(txn *sql.Tx) error) error {
+	if txn == nil {
+		return WithTransaction(db, func(txn *sql.Tx) error {
+			return f(txn)
+		})
+	} else {
+		return f(txn)
+	}
+}