MDEV-7394 : Making slave_skip_errors writable at runtime when slave is stopped

[Mahmoud-kh1]

Now we can update slave_skip_errors at runtime when slaves stopped

• This feature makes the slave_skip_errors system variable dynamic allowing it
  to be changed at runtime when the replication slave is stopped.
• Previously slave_skip_errors was read only at runtime and required a
  server restart to be changed.
• Runtime updates are now validated and safely rejected when the slave is
  running preventing inconsistent replication state.

Key Changes

• Added ON_CHECK handler to verify that updates are only allowed while the
  slave is stopped.
• Added ON_UPDATE handler to reinitialize the internal skip error state
  when the variable is changed.
• Added an rpl mtr test that verifies slave_skip_errors can be changed
  dynamically when the slave is stopped and verified that updates are rejected while the slave is running.

behavior now is like that :

Feature :
MDEV-7394

[CLAassistant]

All committers have signed the CLA.

[Mahmoud-kh1]

The following test cases fail because they assume that slave_skip_errors is read only which is no longer true which making their checks fail also
sys_vars.sysvars_server_notembedded
main.variables-notembedded
sys_vars.slave_skip_errors_basic

[gkodinov]

This is a preliminary review. I'd like to request changes mostly because there are tests that need to be re-recorded (and possibly fixed too).

The rest of the comments are just my own limited take on the change. Feel free to ignore and leave for the final review.

[grooverdan]

Thank you so much for implementing my 11 year old bug report. I'd be very grateful if you stick through the review process on this. There's a lot to keep correct in the server to implement this change.

[gkodinov]

Please just fix the spacing issues. The rest is pretty much as one expects it to be. Aside from some polishing. Once the space changes are gone I will do another round and approve it.

[gkodinov]

Please also address the buildbot failure. It seems related.

[gkodinov]

LGTM. Please do not do changes that are not relevant to the fix (like the one outlined below).

Please stand by for the final review.

[gkodinov]

here and below: why the change?

[Mahmoud-kh1]

I also suspected that the issue might be related to the modifications I made. however, I run the same test on the main branch and it appears the same order.

The test is failing in main branch because the variable is now global and the test has not fixed for that I think (I can make an issue for that and pr to fix it).

so I opened the .opt file where the the initial values is defined and I found it in the same order they appear now

so I suppose it shows the values in the same order as they were set.