lib/db: Use different defaults for larger databases (fixes #5966) (#5967)

This introduces a better set of defaults for large databases. I've
experimentally determined that it results in much better throughput in a
couple of scenarios with large databases, but I can't give any
guarantees the values are always optimal. They're probably no worse than
the defaults though.

lib/config/tuning_test.go

@@ -0,0 +1,26 @@
+// Copyright (C) 2019 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package config_test
+
+import (
+	"testing"
+
+	"github.com/syncthing/syncthing/lib/config"
+	"github.com/syncthing/syncthing/lib/db"
+)
+
+func TestTuningMatches(t *testing.T) {
+	if int(config.TuningAuto) != int(db.TuningAuto) {
+		t.Error("mismatch for TuningAuto")
+	}
+	if int(config.TuningSmall) != int(db.TuningSmall) {
+		t.Error("mismatch for TuningSmall")
+	}
+	if int(config.TuningLarge) != int(db.TuningLarge) {
+		t.Error("mismatch for TuningLarge")
+	}
+}