Workaround for DB lock OperationalError with sqlite3 backend.

An mpi4py test program with the Python sqlite3 wrapper has no trouble
with concurrent DB insertions from several tens of processes.  The issue
appears to be in the Django-sqlite3 wrapper.

This patch simply wraps save() in a loop that intercepts
OperationalError until the save is successful.

balsam/service/models.py

@@ -9,6 +9,7 @@ import uuid
 from django.core.exceptions import ValidationError,ObjectDoesNotExist
 from django.conf import settings
 from django.db import models
+from django.db.utils import OperationalError
 from concurrency.fields import IntegerVersionField
 from concurrency.exceptions import RecordModifiedError
 
@@ -249,7 +250,12 @@ class BalsamJob(models.Model):
             update_fields.append('version')
         if self._state.adding:
             update_fields = None
-        models.Model.save(self, force_insert, force_update, using, update_fields)
+
+        # Work around sqlite3 DB locked error
+        while True:
+            try: models.Model.save(self, force_insert, force_update, using, update_fields)
+            except OperationalError: pass
+            else: break
 
     def __str__(self):
         return f'''