msg: adjust byte_throttler from Message::encode

msg: adjust byte_throttler from Message::encode

Normally we never call encode on a message that has a byte_throttler set
because we only use it for messages we received.  However, for forwarded
messages that we clear_payload() before resending, we *do* reencode, and in
that case we need to retake the appropriate number of bytes from the
throttler--just like we release them in clear_payload().

Signed-off-by: Sage Weil <sage@redhat.com>