Major fixes for forwarding

- Don't increment opaque twice. Each packet is given a new opaque upon
  allocation, so no need to increment this twice (it only leads to
  confusion when reading logs)

- Fix pretty bad bug where values which were contained within the first
  IOV span would end up being chopped off to the header size.

Change-Id: I9ccbed2c6be73488b619343bc052754ac8db82a6
Reviewed-on: http://review.couchbase.org/38286
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>