Add an SQL index on ``envelope_id``

WHERE queries in xbus-odoo's emitter on this column.

models/xbus_emitter_job.py

@@ -111,6 +111,7 @@
         string="Envelope ID",
         help="The ID of the envelope that was forged by xbus-odoo and sent "
         "into the bus",
+        index=True,  # WHERE queries in xbus-odoo's emitter on this column.
     )
 
     process_id = fields.Char(