[IMP] point_of_sale: create one account move by session

Main goal of this task is to create single accounting entry
(AE) when closing a pos.session instead of individual AE for
each order in the session.

This significantly minimizes the number of created journal
entries by point_of_sale which also results to faster closing
of session when there is large numbers (order of thousands)
of orders. It also eases the reconciliation of payments because
on one hand, cash payments are automatically reconciled and on
the other, other payments are combine in a receivable line
based on the payment method.

**Important points to note**

1. Two new models (pos.payment.method and pos.payment) are
introduced replacing the functionalities of account.journal
(and account.bank.statement) and
account.bank.statement.line, which served before as payment
methods and payments.

2. The creation of single AE relies on the receivable_account_id
defined in each pos.payment.method. This receivable_account_id
is supposed to be different from the account module's receivable
account, thus, a new receivable account for pos is introduced
in each localization.

3. A simple example below can illustrate this change.

Given the following orders:

    +---------+----------+----------+-----+-------+-------+
    | order   | payments | product  | qty | price | total |
    +---------+----------+----------+-----+-------+-------+
    | order 1 | cash     | product1 |  10 |    10 |   100 |
    |         |          | product2 |   5 |    20 |   100 |
    +---------+----------+----------+-----+-------+-------+
    | order 2 | bank     | product2 |   7 |    20 |   140 |
    |         |          | product3 |   1 |    30 |    30 |
    +---------+----------+----------+-----+-------+-------+
    | order 3 | bank     | product1 |   1 |    10 |    10 |
    |         |          | product2 |   3 |    20 |    60 |
    |         |          | product3 |   5 |    30 |   150 |
    +---------+----------+----------+-----+-------+-------+

Instead of generating 3 accounting entries, a single
accounting entry (linked to the pos.session) will be created.
This accounting entry will have the following lines:

    +---------------------+---------+------------+
    | account             | balance | reconciled |
    +---------------------+---------+------------+
    | sale                |    -590 |    -       |
    | pos receivable cash |     200 |    yes     |
    | pos receivable bank |     390 |    no      |
    +---------------------+---------+------------+
    | Total balance       |     0.0 |            |
    +---------------------+---------+------------+

Note that the cash receivable line is already reconciled
because it can assumed that the payment is already received.
The unreconciled receivable line can be reconciled manually
using the reconciliation widget.

More examples can be seen in the tests.

TASK-ID: 1862388