jqueue: Add short delay before detecting job changes

By sleeping for 100ms after receiving a notification for a changed job
file the job is given some additional time to change again. This
significantly reduces the number of LUXI calls for WaitForJobChanges
(depending on the job, in my tests with “gnt-cluster verify
--debug-simulate-errors” by about 80%), and improves performance (the
same job went from around 7 seconds to around 3.5 seconds).

This method is not perfect. The algorithm could be made more complex,
e.g. by increasing the delay on each change, etc., but for now this
simple change provides a good improvement.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>