rados.cc: fix listomapvals usage: key,val are not needed

Fixes: #3812
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>
rados.cc: use omap_get_vals_by_keys in getomapval

Fixes: #3811
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>
librados.hpp: fix omap_get_vals and omap_get_keys comments

We list keys greater than start_after.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>
rados.cc: fix rmomapkey usage: val not needed

Signed-off-by: David Zafman <david.zafman@inktank.com>
Reviewed-by: Samuel Just <samuel.just@inktank.com>