denc: add non-contiguous decode_nohead() for bl,string

denc: add non-contiguous decode_nohead() for bl,string

bufferlist's denc traits claims to be need_contiguous=false, so
it should implement all all functions to work with
buffer::list::const_iterator. we already have decode(), the missing
puzzle is decode_nohead().

in this change, decode_nohead(size_t len, bufferlist& v,
                              buffer::list::const_iterator& p) is implemented.

same applies to basic_string.

ideally, we should allow decode buffer::list::iterator as well. but
let's leave it for another change in future when it's needed.

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