Configurable monitor-delay-interval

The `monitor-delay-interval` has been added to the DB config so that
users can change the time period between WAL checks after a file
change notification has occurred. This can be useful to batch up
changes in larger files in the shadow WAL or to reduce or eliminate
the delay in propagating changes during read replication.

Setting the interval to zero or less will disable it.

integration/cmd_test.go

@@ -215,6 +215,55 @@ func TestCmd_Replicate_ResumeWithNewGeneration(t *testing.T) {
 	restoreAndVerify(t, ctx, env, filepath.Join(testDir, "litestream.yml"), filepath.Join(tempDir, "db"))
 }
 
+// Ensure the monitor interval can be turned off.
+func TestCmd_Replicate_NoMonitorDelayInterval(t *testing.T) {
+	ctx := context.Background()
+	testDir, tempDir := filepath.Join("testdata", "replicate", "no-monitor-delay-interval"), t.TempDir()
+	env := []string{"LITESTREAM_TEMPDIR=" + tempDir}
+
+	cmd, stdout, _ := commandContext(ctx, env, "replicate", "-config", filepath.Join(testDir, "litestream.yml"))
+	if err := cmd.Start(); err != nil {
+		t.Fatal(err)
+	}
+
+	db, err := sql.Open("sqlite3", filepath.Join(tempDir, "db"))
+	if err != nil {
+		t.Fatal(err)
+	} else if _, err := db.ExecContext(ctx, `PRAGMA journal_mode = wal`); err != nil {
+		t.Fatal(err)
+	} else if _, err := db.ExecContext(ctx, `CREATE TABLE t (id INTEGER PRIMARY KEY)`); err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	time.Sleep(1 * time.Second)
+
+	// Execute writes periodically.
+	for i := 0; i < 10; i++ {
+		t.Logf("[exec] INSERT INTO t (id) VALUES (%d)", i)
+		if _, err := db.ExecContext(ctx, `INSERT INTO t (id) VALUES (?)`, i); err != nil {
+			t.Fatal(err)
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+
+	// Stop & wait for Litestream command.
+	killLitestreamCmd(t, cmd, stdout)
+
+	// Ensure signal and shutdown are logged.
+	if s := stdout.String(); !strings.Contains(s, `signal received, litestream shutting down`) {
+		t.Fatal("missing log output for signal received")
+	} else if s := stdout.String(); !strings.Contains(s, `litestream shut down`) {
+		t.Fatal("missing log output for shut down")
+	}
+
+	// Checkpoint & verify original SQLite database.
+	if _, err := db.ExecContext(ctx, `PRAGMA wal_checkpoint(TRUNCATE)`); err != nil {
+		t.Fatal(err)
+	}
+	restoreAndVerify(t, ctx, env, filepath.Join(testDir, "litestream.yml"), filepath.Join(tempDir, "db"))
+}
+
 // Ensure the default configuration works with heavy write load.
 func TestCmd_Replicate_HighLoad(t *testing.T) {
 	if testing.Short() {