Bundle an SQL query for performance

We were doing many individual SQL queries for certificates when
awarding program certs. It's possible this was contributing to
high SQL load. Instead, let's bundle those queries up into a larger
one.

LEARNER-6490

openedx/core/djangoapps/programs/utils.py

@@ -284,17 +284,25 @@ class ProgramProgressMeter(object):
 
         Returns a dict of {uuid_string: available_datetime}
         """
+        # Query for all user certs up front, for performance reasons (rather than querying per course run).
+        user_certificates = GeneratedCertificate.eligible_certificates.filter(user=self.user)
+        certificates_by_run = {cert.course_id: cert for cert in user_certificates}
+
         completed = {}
         for program in self.programs:
-            available_date = self._available_date_for_program(program)
+            available_date = self._available_date_for_program(program, certificates_by_run)
             if available_date:
                 completed[program['uuid']] = available_date
         return completed
 
-    def _available_date_for_program(self, program_data):
+    def _available_date_for_program(self, program_data, certificates):
         """
         Calculate the available date for the program based on the courses within it.
 
+        Arguments:
+            program_data (dict): nested courses and course runs
+            certificates (dict): course run key -> certificate mapping
+
         Returns a datetime object or None if the program is not complete.
         """
         program_available_date = None
@@ -305,7 +313,7 @@ class ProgramProgressMeter(object):
                 key = CourseKey.from_string(course_run['key'])
 
                 # Get a certificate if one exists
-                certificate = GeneratedCertificate.eligible_certificates.filter(user=self.user, course_id=key).first()
+                certificate = certificates.get(key)
                 if certificate is None:
                     continue