cpg: Add support for messages larger than 1Mb

If a cpg client sends a message larger than 1Mb (actually slightly
less to allow for internal buffers) cpg will now fragment that into
several corosync messages before sending it around the ring.

cpg_mcast_joined() can now return CS_ERR_INTERRUPT which means that the
cpg membership was disrupted during the send operation and the message
needs to be resent.

The new API call cpg_max_atomic_msgsize_get() returns the maximum size
of a message that will not be fragmented internally.

New test program cpghum was written to stress test this functionality,
it checks message integrity and order of receipt.

Signed-off-by: Christine Caulfield <ccaulfie@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>