Persist partition|offset|user_id in the keyserver (#1226)

* Persist partition|offset|user_id in the keyserver

Required for a query API which will be used by the syncapi which
will be called when a `/sync` request comes in which will return
a list of user IDs of people who have changed their device keys
between two tokens.

* Add tests and fix maxOffset bug

* s/offset/log_offset/g because 'offset' is a reserved word in postgres

keyserver/storage/postgres/storage.go

@@ -34,9 +34,14 @@ func NewDatabase(dbDataSourceName string, dbProperties sqlutil.DbProperties) (*s
 	if err != nil {
 		return nil, err
 	}
+	kc, err := NewPostgresKeyChangesTable(db)
+	if err != nil {
+		return nil, err
+	}
 	return &shared.Database{
 		DB:               db,
 		OneTimeKeysTable: otk,
 		DeviceKeysTable:  dk,
+		KeyChangesTable:  kc,
 	}, nil
 }