Patch for BUG#31111: --read-only crashes MySQL (events fail to load).

There actually were several problems here:
  - WRITE-lock is required to load events from the mysql.event table,
    but in the read-only mode an ordinary user can not acquire it;
  - Security_context::master_access attribute was not properly
    initialized in Security_context::init(), which led to differences
    in behavior with and without debug configure options.
  - if the server failed to load events from mysql.event, it forgot to
    close the mysql.event table, that led to the coredump, described
    in the bug report.

The patch is to fix all these problems:
  - Use the super-user to acquire WRITE-lock on the mysql.even table;
  - The WRITE-lock is acquired by the event scheduler in two cases:
    - on initial loading of events from the database;
    - when an event has been executed, so its attributes should
      be updated.
    Other cases when WRITE-lock is needed for the mysql.event table
    happen under the user account. So, nothing should be changed there
    for the read-only mode. The user is able to create/update/drop
    an event only if he is a super-user.
  - Initialize Security_context::master_access;
  - Close the mysql.event table in case something went wrong.

sql/sql_class.cc

@@ -2404,6 +2404,7 @@ void Security_context::init()
   host= user= priv_user= ip= 0;
   host_or_ip= "connecting host";
   priv_host[0]= '\0';
+  master_access= 0;
 #ifndef NO_EMBEDDED_ACCESS_CHECKS
   db_access= NO_ACCESS;
 #endif