Add database indices

Based on review of WHERE clauses in DAO queries

Partially address recent downtime issue due to DB connection pool saturation

src/main/sql/updates.sql

@@ -168,4 +168,12 @@ alter table job add column user_agent VARCHAR (400);
 
 
 -- 2.6.0
-alter table `user` add column api_token_expires_on timestamp null default null;
+alter table `user` add column api_token_expires_on timestamp null default null;
+
+-- 2.7.1
+CREATE INDEX idx_downloads_hash ON downloads(hash);
+CREATE INDEX idx_downloads_path ON downloads(path);
+
+CREATE INDEX idx_user_username ON user(username);
+CREATE INDEX idx_user_mail ON user(mail);
+CREATE INDEX idx_user_fullname ON user(full_name);