LU-1943 tests: async IO is enough for sanity/80

Port of ORI-620 to the master

it can be hard to write 1MB in a second for some
backends (like ZFS), so relax current synchronous
semantics as this doesn't affect the logic.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Change-Id: Ie87815de4a2ae51fc166287d74d43278101fe76b
Reviewed-on: http://review.whamcloud.com/4212
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>