Feature request: introduce option to set the expiration time of the *alive* keys in the setOK thread

[telenskyt]

Sometimes the tasks get resubmitted without reason (see #14). On my very overloaded system it happens quite often. It can perhaps be caused by quite short expiration time of the alive keys (default 5 seconds now). With the 3 seconds periond for refreshing the keys, on a very overloaded system it might happen that the key will expire.

It would be very useful to be able to set my own timeout for the alive key (and perhaps also for the refreshing period (3 seconds now)).