denc: add need_contiguous to denc_traits

denc: add need_contiguous to denc_traits

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
CEPH_PAGE_SIZE (4K) if the decoded object does not "need_contiguous" per
its denc_traits<>.

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.

also use denc() in interval_set<>::decode(). unlike encode(), decode(),
encode_nohead() or decode_nohead(), denc() is not part of our legacy
dencoder, so i think it's fine and encouraged to use it.

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