MDEV-33145 Add FLUSH GLOBAL STATUS

- FLUSH GLOBAL STATUS now resets most global_status_vars.
  At this stage, this is mainly to be used for testing.
- FLUSH SESSION STATUS added as an alias for FLUSH STATUS.
- FLUSH STATUS does not require any privilege (before required RELOAD).
- FLUSH GLOBAL STATUS requires RELOAD privilege.
- All global status reset moved to FLUSH GLOBAL STATUS.
- Replication semisync status variables are now reset by
  FLUSH GLOBAL STATUS.
- In test cases, the only changes are:
  - Replace FLUSH STATUS with FLUSH GLOBAL STATUS
  - Replace FLUSH STATUS with FLUSH STATUS; FLUSH GLOBAL STATUS.
    This was only done in a few tests where the test was using SHOW STATUS
    for both local and global variables.
- Uptime_since_flush_status is now always provided, independent if
  ENABLED_PROFILING is enabled when compiling MariaDB.
- @@global.Uptime_since_flush_status is reset on FLUSH GLOBAL STATUS
  and @@session.Uptime_since_flush_status is reset on FLUSH SESSION STATUS.
- When connected, @@session.Uptime_since_flush_status is set to 0.

sql/sql_class.cc

@@ -1710,6 +1710,7 @@ void THD::reset_for_reuse()
   m_command= COM_CONNECT;
   proc_info= "login";                           // Same as in THD::THD()
   transaction->on= 1;
+  status_var.flush_status_time= my_time(0);
 #if defined(ENABLED_PROFILING)
   profiling.reset();
 #endif
@@ -4560,6 +4561,7 @@ void THD::set_status_var_init()
     STATUS. And at this point thread is guaranteed to be running.
   */
   status_var.threads_running= 1;
+  status_var.flush_status_time= my_time(0);
 }