analytics: Change update_analytics_counts to only use hour boundaries.

Fixes a recent regression where analytics were not being run on hour
boundaries.

Includes a migration that dumps all the analytics data.

analytics/migrations/0011_clear_analytics_tables.py

@@ -0,0 +1,29 @@
+# -*- coding: utf-8 -*-
+from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+from django.db import migrations
+
+
+def clear_analytics_tables(apps, schema_editor):
+    # type: (StateApps, DatabaseSchemaEditor) -> None
+    UserCount = apps.get_model('analytics', 'UserCount')
+    StreamCount = apps.get_model('analytics', 'StreamCount')
+    RealmCount = apps.get_model('analytics', 'RealmCount')
+    InstallationCount = apps.get_model('analytics', 'InstallationCount')
+    FillState = apps.get_model('analytics', 'FillState')
+
+    UserCount.objects.all().delete()
+    StreamCount.objects.all().delete()
+    RealmCount.objects.all().delete()
+    InstallationCount.objects.all().delete()
+    FillState.objects.all().delete()
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('analytics', '0010_clear_messages_sent_values'),
+    ]
+
+    operations = [
+        migrations.RunPython(clear_analytics_tables),
+    ]