blktrace: add option to scale a trace

blktrace: add option to scale a trace

As we explore stacking traces, it is nice to be able to scale a trace to
understand how the traces end up interacting.

This patch adds scaling by letting the user pass in percentages to scale
a trace by. When passed '--merge_blktrace_scalars="100"', the trace is
ran at 100% speed. If passed 50%, this will halve the trace timestamps.
The new option takes in a comma separated list that index-wise pairs
with the passed files in "--read_iolog".

This option differs from "--replay_time_scale" which scales the trace
during runtime and will not change the output unlike this option.

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