In case an alias is configured to fecth email to generate
account entries. the new_message method directly call the
_compute_name on account move which call the legacy code
provide by account odoo module which should be avoid here.
To reproduce:
* configure an alias on your purchase journal
* send an email to that alias
* the invoice is draft with a number
to mitigate this behavior the suggested patch overwritte
the former method '_compute_name' and call the new implemented
method '_compute_name_by_sequence'.
A demo ir_sequence record with 'standard' implementation was
assigned to the payment journal so that no concurrency errors arise
from using 'no gap' sequences when attempting to create multiple payments.