[FIX] account_financial_report: Avoid slow vacuum due to ondelete=cascade

`report_journal_ledger` is auto-vacuumed as any transient model, but has some
ondelete="cascade" constraints that auto-remove subtables when a record is removed,
doing this operation very slow when selecting these sub-records.

Letting default ondelete="set null" would result in same performance bottleneck,
as the select on sub-table is performed the same for setting "null" value on them.

As a solution, and for avoiding a costly index operation, we delete by SQL sub-tables
rows in advance.

A bit of extra logic has been added for avoiding to remove that records if it's not
the turn of vacuum the parent table.

account_financial_report/readme/CONTRIBUTORS.rst

@@ -16,6 +16,10 @@
 * Mihai Fekete <feketemihai@gmail.com>
 * Miquel Raïch <miquel.raich@eficent.com>
 * Isaac Gallart <igallart@puntsistemes.com>
+* `Tecnativa <https://www.tecnativa.com>`__:
+
+  * Pedro M. Baeza
+  * Sergio Teruel
 
 Much of the work in this module was done at a sprint in Sorrento, Italy in
 April 2016.

account_financial_report/report/journal_ledger.py

@@ -612,6 +612,19 @@ class ReportJournalLedger(models.TransientModel):
     def get_html(self, given_context=None):
         return self._get_html()
 
+    @api.model
+    def _transient_vacuum(self, force=False):
+        """Remove journal ledger subtables first for avoiding a costly
+        ondelete operation.
+        """
+        # Next 3 lines adapted from super method for mimicking behavior
+        cls = type(self)
+        if not force and (cls._transient_check_count < 21):
+            return True  # no vacuum cleaning this time
+        self.env.cr.execute("DELETE FROM report_journal_ledger_move_line")
+        self.env.cr.execute("DELETE FROM report_journal_ledger_move")
+        return super(ReportJournalLedger, self)._transient_vacuum(force=force)
+
 
 class ReportJournalLedgerJournal(models.TransientModel):