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>
It's unsafe to move this unlock here. session_security needs to be read while the pipe_lock is held - see 1d74170a4c252f35968ccfbec8e432582e92f638 and http://tracker.ceph.com/issues/6480