denc: add with_dencv to denc_traits

the "v" in dencv is like the "v" in readv(2), so the decode() will not
try to copy the bufferlist for a continous one if the bufferlist is
segmented *and* its length is greater than a CEPH_PAGE_SIZE (4K).

copying a memory chunk could be expensive if the decoded bufferlist is
huge, so we should try to avoid this. this could happen when we read the
buffer from bluestore.

and drop the partial specialization for denc() which tries to differentiate
traits::featured and !traits::featured, it does not matter to
decode() if the type supports feature or not. the encode() does.

Signed-off-by: Kefu Chai <kchai@redhat.com>