Set transaction mode to "READ COMMITTED" to avoid deadlocks. Bug 1492919

behatnotneeded: Testing requires multiple simultaneous users

Change-Id: Icfac83b0ac9ed6ab0c304658550dd0260bd5d7eb
(cherry picked from commit 0eea42ce)

htdocs/lib/dml.php

@@ -1473,6 +1473,8 @@ function configure_dbconnection() {
         $db->Execute("SET SQL_BIG_SELECTS=1");
     }
 
+    $db->SetTransactionMode('READ COMMITTED');
+
     if (!empty($CFG->dbtimezone)) {
         if (is_postgres()) {
             $db->Execute("SET SESSION TIME ZONE '{$CFG->dbtimezone}'");