LP #1612650: audit_log plugin contention on libc TZ conversion

**Reported in Launchpad by Laurynas Biveinis last update 12-08-2016 14:25:43

PMP on a server with audit_log enabled shows the following aggregated stacktrace:

113 _lll_lock_wait_private(libc.so.6),_tz_convert(libc.so.6),audit_log_general_record(audit_log.so),audit_log_notify(audit_log.so),general_class_handler,mysql_audit_notify,dispatch_command,do_handle_one_connection,handle_one_connection,pfs_spawn_thread,start_thread(libpthread.so.0),clone(libc.so.6)

This is caused by UTC timestamp printing in make_timestamp function, the sequence of libc calls time(NULL) -> gmtime_r -> strftime.