process: Introduce notify_job_request()

process: Introduce notify_job_request()

This function can be used to kick the main thread out of
poll_job_request() function in case someone wants to re-schedule rrd
processing.

As a starter, a notify call is added at the end of worker processing
when the last worker thread exits. The only purpose of this feature is
to make the main thread to print out the time of the next scheduled
job to be processed, making it just easier to follow the daemon as it
operates.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>