lib/db, lib/model: Add sequence->deviceKey to db for sending indexes (#4906)

Instead of walking and unmarshalling the entire db and sorting the resulting
file infos by sequence, add store device keys by sequence number in the
database. Thus only the required file infos need be unmarshalled and are already
sorted by index.

lib/db/leveldb.go

@@ -11,11 +11,9 @@ import (
 	"fmt"
 
 	"github.com/syncthing/syncthing/lib/protocol"
-	"github.com/syndtr/goleveldb/leveldb"
-	"github.com/syndtr/goleveldb/leveldb/opt"
 )
 
-const dbVersion = 1
+const dbVersion = 2
 
 const (
 	KeyTypeDevice = iota
@@ -29,6 +27,7 @@ const (
 	KeyTypeIndexID
 	KeyTypeFolderMeta
 	KeyTypeMiscData
+	KeyTypeSequence
 )
 
 func (l VersionList) String() string {
@@ -60,28 +59,5 @@ func (l fileList) Less(a, b int) bool {
 	return l[a].Name < l[b].Name
 }
 
-type dbReader interface {
-	Get([]byte, *opt.ReadOptions) ([]byte, error)
-}
-
 // Flush batches to disk when they contain this many records.
 const batchFlushSize = 64
-
-func getFile(db dbReader, key []byte) (protocol.FileInfo, bool) {
-	bs, err := db.Get(key, nil)
-	if err == leveldb.ErrNotFound {
-		return protocol.FileInfo{}, false
-	}
-	if err != nil {
-		l.Debugln("surprise error:", err)
-		return protocol.FileInfo{}, false
-	}
-
-	var f protocol.FileInfo
-	err = f.Unmarshal(bs)
-	if err != nil {
-		l.Debugln("unmarshal error:", err)
-		return protocol.FileInfo{}, false
-	}
-	return f, true
-}