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>
I don't think this is the sort of setting where preventing users from killing themselves is actually that helpful. 128KB is probably enough for many users to kill themselves anyway, but it may not be large enough for people on ultra-fast networking with high-performance systems and bulk data transfers.