[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

@@ -15,6 +15,10 @@
 * Alexis de Lattre <alexis@via.ecp.fr>
 * Mihai Fekete <feketemihai@gmail.com>
 * Miquel Ra??ch <miquel.raich@eficent.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.