Allow PRIORITY frame at anytime.

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.
Assign default weight if NGHTTP2_MAX_DEP_TREE_LENGTH is exceeded
Make certain type of HEADERS subject to priority

We make following HEADERS under priority control:
* push response HEADERS
* HEADERS submitted by nghttp2_submit_response

Currently, HEADERS submitted by nghttp2_submit_headers is not attached
to stream.  This is because it may be used as non-final response
header and application may submit final response using
nghttp2_submit_response without checking non-final response header
transmission.
Reduce nghttp2_session_get_stream() call before sending frame
Update doc
Remove duplicated code
Handle idle stream in priority field