mon_thrash: use _ instead of - consistently

Signed-off-by: Sage Weil <sage@inktank.com>
mon_thrash: fix more naming

Signed-off-by: Sage Weil <sage@inktank.com>
mon_thrash: optionally scrub after each iteration (default true)

Signed-off-by: Sage Weil <sage@inktank.com>