MINOR: mworker: simplify the code that sets PROC_O_LEAVING

MINOR: mworker: simplify the code that sets PROC_O_LEAVING

When master performs a reexec it should set for an already existed worker the
flag PROC_O_LEAVING. It means that existed worked is marked as the previous one
and will be terminated after the reload.

In the previous implementation master process was need to do the reexec
twice (the first time for parsing its configuration and the second time to free
unused ressources). So the logic of setting PROC_O_LEAVING was based on
comparing the number of reloads, performed by each process from the processes
list, except the master.

Now, as being mentioned before, reexec is performed only once. So, in this case
we need to set PROC_O_LEAVING flag, when we deserialize the list. It is done for
all processes, which have the number of reloads stricly positive.