Implement addBatchInDisk for big trees

Implement addBatchInDisk for big trees, which does not puts the tree in
memory, and works directly over the db data, parallelizing for each CPU.

testvectors/circom/go-data-generator/generator_test.go

@@ -8,12 +8,13 @@ import (
 
 	qt "github.com/frankban/quicktest"
 	"github.com/vocdoni/arbo"
+	"go.vocdoni.io/dvote/db"
 	"go.vocdoni.io/dvote/db/badgerdb"
 )
 
 func TestGenerator(t *testing.T) {
 	c := qt.New(t)
-	database, err := badgerdb.New(badgerdb.Options{Path: c.TempDir()})
+	database, err := badgerdb.New(db.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
 	tree, err := arbo.NewTree(database, 4, arbo.HashFunctionPoseidon)
 	c.Assert(err, qt.IsNil)