Allowing PRIORITY frame at anytime so that PRIORITY frame to idle
stream can create anchor node in dependency tree.  In this change, we
open stream with new NGHTTP2_STREAM_IDLE state, which is linked in
session->closed_stream_head and is treated as if it is closed stream.
One difference is that if the stream is opened, we remove it from
linked list and change the state to the appropriate one.  To O(1)
removal from linked list, we change session->closed_stream_head to
doubly linked list.