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.go

@@ -0,0 +1,47 @@
+// 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
+
+type Tuning int
+
+const (
+	// N.b. these constants must match those in lib/db.Tuning!
+	TuningAuto Tuning = iota // default is auto
+	TuningSmall
+	TuningLarge
+)
+
+func (t Tuning) String() string {
+	switch t {
+	case TuningAuto:
+		return "auto"
+	case TuningSmall:
+		return "small"
+	case TuningLarge:
+		return "large"
+	default:
+		return "unknown"
+	}
+}
+
+func (t Tuning) MarshalText() ([]byte, error) {
+	return []byte(t.String()), nil
+}
+
+func (t *Tuning) UnmarshalText(bs []byte) error {
+	switch string(bs) {
+	case "auto":
+		*t = TuningAuto
+	case "small":
+		*t = TuningSmall
+	case "large":
+		*t = TuningLarge
+	default:
+		*t = TuningAuto
+	}
+	return nil
+}