[FIX] account: bank statement confirm performance improvement
[FIX] account: bank statement confirm performance improvement

`fast_counterpart_creation` creates new move lines.
The ORM doesn't know if these new move lines changed
or not the value of the field `journal_entry_ids` of each
of the bank statement lines, and for that reason
this field got invalidated from the cache at each iteration,
and the value was prefetched as soon as
`st_line.journal_entry_ids.ids` was asked.

Storing the value of this field for each line in the dict
`journal_entries` overcome this problem as we get the value
from this dict during the loop.
Of course, this relies on the fact the value of `journal_entry_ids`
doesn't change during the loop
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