Use 'chunk' option to bulkImport

Currently, we use a custom loop to bulkImport transactions. The typeorm API provides a chunk option to save methods (which also can bulk insert entities. This also allows to return the created data.

see: https://github.com/typeorm/typeorm/blob/master/docs/repository-api.md#additional-options

I have tested, it works fine.

Do we want to generalize the bulkCreate API to all models ? This would speed up imports, and fetching new transactions/accounts.