[FIX] account: reduce locking on moves

By trying to avoid concurrency issues while assigning the sequence [1],
we are locking too many operations even when not in the process of
assigning a new number.
* when computing `highest_name`, we don't need to lock as this is mostly
  an information on the form that can possibly change anyway while the
  form view is open (without any lock anymore)
* when computing the accounting date, we don't need to lock, the only
  information we want is the format of the sequence (yearly, monthly, no
  reset), which is not likely to change on a same journal.
* when changing the `journal_id`, it is only a helper for encoding, if
  there is a concurrency error it will be shown during the post anyways.
  Also same argument as for `highest_name`

[1] https://github.com/odoo/odoo/commit/ba6ee0b75d66d6f22a592b1b6a7c5158461bec41



closes odoo/odoo#94852

Related: odoo/enterprise#28957
Signed-off-by: Olivier Colson (oco) <oco@odoo.com>