extend rollupDB

[krlosMata]

RollupDB should be extended to fully support massive-migrations.
Therefore, the following procedures should be implemented:

processBatchMigrate: mimics migrate circuit logic to insert new leafs into the state-tree

Possible implementation

Add new function in rollupDB: buildBatchMigration

[krlosMata]

This has been implemented partially in order to perform circuit test in https://github.com/hermeznetwork/circuits/blob/feature/upgrade-state-root/js/upgrade-db.js

[krlosMata]

This has been coded in https://github.com/hermeznetwork/commonjs/tree/feature/updateDb
note: test are missing