Have ability for event observers to defer firing until after the sessions is closed

BEFORE PATCH 1) Hack the standard log store to mimic it being expensive: +++ b/admin/tool/log/store/standard/classes/log/store.php @@ -66,6 +66,12 @@ class store implements \tool_log\log\writer, \core\log\sql_internal_table_reader protected function insert_event_entries( $evententries ) { global $DB ; + for ( $c =1; $c <5; $c ++) { + error_log ( "waiting $c" ); + sleep(1); + flush (); +} + $DB ->insert_records( 'logstore_standard_log' , $evententries ); 2) Visit any page which emits a log, such as the live logs page itself: http://moodle.local/report/loglive/index.php 3) Confirm it takes ~5 seconds to load 4) Quickly open up this url in 5 tabs, note that they block each other and will take ~25 seconds overall until the last one load. AFTER PATCH 5) Repeat the loading 5 tabs and confirm that each pages takes 5 seconds but they load in parallel.