taskd.pl: implement --max-lifetime with 1 week default
When taskd's maximum lifetime (in realtime) is exceeded, initiate
a graceful restart. The default is one week.
A graceful restart of taskd will trigger a graceful restart of
jobd if jobd is running at the time the graceful restart occurs.
While neither taskd nor jobd are known to be prone to memory bloat
(where the memory used continues to increase the longer they run),
a maximum lifetime setting provides a nice safety valve to prevent
such a problem from occurring in the first place.
A maximum lifetime setting of 0 can be used to disable this feature.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>