Log when starting and ending task exclusive mode
requested to merge 3137-create-new-log-messages-when-entering-and-exiting-exlusive-task-mode-v9_16 into v9_16
The task exclusive mode stops all processing (tasks and networking IO) except the designated exclusive task events. This has impact on the operation of the server. Add log messages indicating when we start the exclusive mode, and when we end exclusive task mode.
(cherry picked from commit b9cb2907)
Closes #3137 (closed)