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/syncthing/utils.go

@@ -122,6 +122,6 @@ func copyFile(src, dst string) error {
 	return nil
 }
 
-func OpenGoleveldb(path string) (*db.Lowlevel, error) {
-	return db.Open(path)
+func OpenGoleveldb(path string, tuning config.Tuning) (*db.Lowlevel, error) {
+	return db.Open(path, db.Tuning(tuning))
 }