[Merge addon] TSN-R does not cleanup admin_mrgclient in case if source client is missing

[avsaponchik]

Topic name.
If, somehow, you manage to slow-down your ranksystem and spam it with the merge tasks and get the same merge request twice or more then there will be warnings in the log file, after which the table will retain the records and will not be cleaned after that.



after the check, if source client is missing there should be a warning in the logfile and such task should be removed from the table.

[avsaponchik]

I'm keeping those in the table to test this again after you make some changes to the code.