msg: prefetch data when doing recv()

Since that when we're reading the header we do lot's of small IOs, we
can really benefit from doing bigger reads.
Set a limit on the buffer size, so that it should mainly affect header
reads and not the actual data transfer of large IOs.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #2457 from ceph/wip-7767-b

msg: prefetch data when doing recv()

Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktnak.com>