iolog: run compression work at slightly elevated nice level

Add support for work items to contain a priority, and use this
directly with nice. Run compression at nice 1, to make it slightly
less important than the actual IO jobs.

Signed-off-by: Jens Axboe <axboe@fb.com>
Add thread number to log filename

Since commit aee2ab6775d9, we open the log files for writing and
truncate the output. This breaks when you have multiple jobs with
the same name, as will happen with numjobs=x. Instead of reverting
this part of the commit, lets keep the truncate open and instead
store log files separately. This makes more sense anyway, since
otherwise the logs are all the entries from job X, then all the
entries from job Y, etc. It's easy enough to collate the logs,
should anyone want to do that.

Signed-off-by: Jens Axboe <axboe@fb.com>