When set the server-id dynamically, the server_id member of current thread is not updated.

Update the server_id member of current thread after updated the global variable value.


sql/set_var.cc:
  Update server_id of current thread
mysql-test/r/rpl_server_id.result:
  Add test for BUG#28908
mysql-test/t/rpl_server_id.test:
  Add test for BUG#28908