blktrace: add support to interleave blktrace files

blktrace: add support to interleave blktrace files

Running concurrent workloads via multiple jobs can lead to
nondeterministic results as we are at the scheduler's mercy. While the
actual performance of the workload may vary regardless, this makes the
order of events consistent.

This patch introduces two flags: --merge_blktrace_file=<file> and
--merge-blktrace-only. When the first is specified, files that are ':'
separated in --read_iolog are passed to a merge function wich then
produces a single sorted trace file. This file is then passed on in
place of the sorted list or fio exists if --merge-blktrace-only is
specified.

During merging, events are filtered based on what fio cares about and
the pdu is discarded as well.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>