Refactor StoreEvent, add MaybeRedactEvent, create an EventDatabase (#2989)

This PR changes the following:
- `StoreEvent` now only stores an event (and possibly prev event),
instead of also doing redactions
- Adds a `MaybeRedactEvent` (pulled out from `StoreEvent`), which should
be called after storing events
- a few other things

roomserver/roomserver_test.go

@@ -278,6 +278,16 @@ func TestPurgeRoom(t *testing.T) {
 		if roomInfo == nil {
 			t.Fatalf("room does not exist")
 		}
+
+		//
+		roomInfo2, err := db.RoomInfoByNID(ctx, roomInfo.RoomNID)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !reflect.DeepEqual(roomInfo, roomInfo2) {
+			t.Fatalf("expected roomInfos to be the same, but they aren't")
+		}
+
 		// remember the roomInfo before purging
 		existingRoomInfo := roomInfo
 
@@ -333,6 +343,10 @@ func TestPurgeRoom(t *testing.T) {
 		if roomInfo != nil {
 			t.Fatalf("room should not exist after purging: %+v", roomInfo)
 		}
+		roomInfo2, err = db.RoomInfoByNID(ctx, existingRoomInfo.RoomNID)
+		if err == nil {
+			t.Fatalf("expected room to not exist, but it does: %#v", roomInfo2)
+		}
 
 		// validation below