messages/MForward: fix encoding features

messages/MForward: fix encoding features

We were encoding the message with the sending client's
features, which makes no sense: we need to encode with
the recipient's features so that it can decode the
message.

The simplest way to fix this is to rip out the bizarre
msg_bl handling code and simply keep a decoded Message
reference, and encode it when we send.

We encode the encapsulated message with the intersection
of the target mon's features and the sending client's
features.  This probably doesn't matter, but it's
conceivable that there is some feature-dependent
behavior in the message encode/decode that is important.

Fixes: http://tracker.ceph.com/issues/17365
Signed-off-by: Sage Weil <sage@redhat.com>