msg/simple: ms_die_on_skipped_message

Assert if we skip an incoming seq.  The SimpleMessenger never does this,
but the kernel messenger does.  Hopefully useful for tracking down bugs
like #10057.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2916 from ceph/wip-10057

msg/simple: ms_die_on_skipped_message

Reviewed-by: Greg Farnum <greg@inktank.com>