adding `chirp_time` as index for `avls_data`

server/sql_schema/create_tables.sql

@@ -30,7 +30,9 @@ CREATE TABLE IF NOT EXISTS avls_data (
   latitude DOUBLE,
   longitude DOUBLE,
   heading DOUBLE,
-  velocity DOUBLE
+  velocity DOUBLE,
+
+  INDEX chirp_idx (chirp_time)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
 
 -- Bus Pass Table holds all bus pass holders