[FIX] point_of_sale: fixed errors that avoid closing a pos session + [IM...

...P] account: added back the account_id field on statement.line obj to ease the reconciliation process when importing statements (or creating them through pos.orders). Fix #932

i added back the 'account_id' field on bank.statement.line but that shouldn't be considered as a database schema change since it was there in saas 4.