objectcacher: Remove commit_set, use flush_set

commit_set() and flush_set() are identical in functionality,
so use flush_set everywhere and remove commit_set from
the code.

Also fixes a bug in flush_set where the finisher context was
getting freed twice if no objects needed to be flushed.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
qa/workunit: Add iozone test script for sync

The iozone-sync.sh script runs iozone testing
various sync flags, O_SYNC, O_DSYNC, O_RSYNC.

Signed-off-by: Sam Lang <sam.lang@inktank.com>