Add netbuf packet allocator/write queuer

This adds the netbuf packet allocator. This is currently unused but will
be employed extensively as we transition away from ringbuffer write
queues to an IOV-based system

Change-Id: Ic623732cc03052a9e25bb2696a796875a0313d18
Reviewed-on: http://review.couchbase.org/33947
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>