manager: merge serialization and desrialization counter into one, and increase it...

manager: merge serialization and desrialization counter into one, and increase it when reexecuting

Instead of having individual counters n_serializing and n_deserializing
have a single one n_reloading, which should be sufficient.

Set n_reloading when we are about to go down for reexecution to avoid
cgroup trimming when we free the units for reexecution.