reload: wait for pending tasks to submit and pause the workflow

* Closes cylc#5107
* Reload now waits for pending tasks to submit before attempting to
  reload the config itself.
* Reload now also puts the workflow into the paused state during the
  reload process. This doesn't actually achieve anything as the
  reload command is blocking in the main loop, but it does help to
  communicate that the workflow will not de-queue or submit and new
  tasks during this process.
* The workflow status message is now updated to reflect the reload
  progress.

CHANGES.md

@@ -14,6 +14,10 @@ ones in. -->
 
 ### Enhancements
 
+[#5992](https://github.com/cylc/cylc-flow/pull/5992) -
+The scheduler will now wait for preparing tasks to submit before attempting
+to perform a reload and will also pause/unpause the workflow.
+
 [#5405](https://github.com/cylc/cylc-flow/pull/5405) - Improve scan command
 help, and add scheduler PID to the output.