tools: fix race condition in seq/rand bench

tools: fix race condition in seq/rand bench

Under certain conditions (like bench seq/rand -b 1024 -t 128) it is
possible that aio_read reads data into destination buffers before or
during memcmp execution, resulting in "[..] is not correct!" errors
even if actual objects are perfectly fine.
Also, moved latencty calculation around, so it is no longer affeted
by memcmp.

Signed-off-by: Piotr Dałek <piotr.dalek@ts.fujitsu.com>
Conflicts:
	src/common/obj_bencher.cc