Create index on user_ips in the background

user_ips is kinda big, so really we want to add the index in the background
once we're running. Replace the schema delta with one which will do that.

I've done this in a way that's reasonably easy to reuse as there a few other
indexes I need, and I don't suppose they will be the last.

synapse/storage/client_ips.py

@@ -15,10 +15,11 @@
 
 import logging
 
-from ._base import SQLBaseStore, Cache
-
 from twisted.internet import defer
 
+from ._base import Cache
+from . import background_updates
+
 logger = logging.getLogger(__name__)
 
 # Number of msec of granularity to store the user IP 'last seen' time. Smaller
@@ -27,8 +28,7 @@ logger = logging.getLogger(__name__)
 LAST_SEEN_GRANULARITY = 120 * 1000
 
 
-class ClientIpStore(SQLBaseStore):
-
+class ClientIpStore(background_updates.BackgroundUpdateStore):
     def __init__(self, hs):
         self.client_ip_last_seen = Cache(
             name="client_ip_last_seen",
@@ -37,6 +37,14 @@ class ClientIpStore(SQLBaseStore):
 
         super(ClientIpStore, self).__init__(hs)
 
+        self.register_background_index_update(
+            "user_ips_device_index",
+            index_name="user_ips_device_id",
+            table="user_ips",
+            columns=["user_id", "device_id", "last_seen"],
+        )
+
+
     @defer.inlineCallbacks
     def insert_client_ip(self, user, access_token, ip, user_agent, device_id):
         now = int(self._clock.time_msec())