Allow the new bank statement reconciliation to search in transaction_ref

**Work in progress** because a merge is needed in the core to be able to inherit the methods.

 * [x] https://github.com/odoo/odoo/pull/3025 to be merged to work!
 * [x] Also, the migration of `base_transaction_id` (pre-requisite before adding features) is there: https://github.com/OCA/bank-statement-reconcile/pull/47