presence: Add realm/timestamp index to UserPresence.

It adds this index:

    "zerver_userpresence_realm_id_timestamp_25f410da_idx" btree (realm_id, "timestamp")

We expect this index to provide a major performance improvement when
fetching presence data for the whole realm from the database on
servers like zulipchat.com hosting several realms.

zerver/migrations/0268_add_userpresence_realm_timestamp_index.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.28 on 2020-02-08 20:34
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0267_backfill_userpresence_realm_id'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='userpresence',
+            name='realm',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='zerver.Realm'),
+        ),
+        migrations.AlterIndexTogether(
+            name='userpresence',
+            index_together=set([('realm', 'timestamp')]),
+        ),
+    ]

zerver/models.py

@@ -2327,9 +2327,12 @@ class UserPresence(models.Model):
     """
     class Meta:
         unique_together = ("user_profile", "client")
+        index_together = [
+            ("realm", "timestamp")
+        ]
 
     user_profile = models.ForeignKey(UserProfile, on_delete=CASCADE)  # type: UserProfile
-    realm = models.ForeignKey(Realm, null=True)  # type: Optional[Realm]
+    realm = models.ForeignKey(Realm)  # type: Realm
     client = models.ForeignKey(Client, on_delete=CASCADE)  # type: Client
 
     # The time we heard this update from the client.