DirtyCleanInterval should be 0 by default

Please consider changing DirtyCleanInterval value to 0 as default.

Otherwise if cupsd crashes due to (e.g. OOM killer) under a heavy workload even hundreds of jobs may be lost. This concern is backed up by a real-life scenario and leaves the client sending thousands of jobs unaware that many of them are lost during a crash.

After cupsd gets restarted it rewinds it's job counter to the last cached and continues unaware about the jobs accepted and lost.

Having DirtyCleanInterval set to 0 will cause some performance impact, but not significant under lighter workloads and a completely justified price for reliability under heavy workloads.

Test scenario:
1. sudo apt install printer-driver-cups-pdf
2. while [ 1 ]; do lp -d PDF somepdf.pdf; done;
3. # on other terminal
   kill -9 $(pidof cupsd)
4. Note last job number and wait for cupsd to be restarted by systemd.
5. Once accepting jobs is resumend the job counter is rewound.

Expected behavior:
Accepted jobs are queued for processing.

Actual behavior:
Some accepted jobs are lost.