Convert rrdtool to use work queues instead of forks
Switch to the work queue model. Change is somewhat trivial. The
database update timestamp update is now done before the actual
update. We don't have a mean to distinguis from the fact that we have
initiated an update but not done it yet, so we mark it as "done" and
then queue the update. Without this change the main thread will keep
on queueing the update indefinitely as the worker thread takes too
long to actually mark the update as done.
To ensure the queues work properly, we also need to disable sigchld
handler from the process controller so that it does not get confused
when threads are reaping the childs them selves. This is different
from the previous model where each process required to handle their
own child reaping after fork(). Now we are sharing the signals with
the parent. This is a major simplification on how this works.
The magic sleep on job control init can be removed as well as there
are less forks going on now.
Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>