Combine deferred_data and data into data_item and merge deferred_flags
into flags.

This is preliminary change for upcoming HPACK updates.  The flags are
used to determine the name/value pair is indexable or not.

We do -+1 on serialization and deserialization since the field is 1
byte.  This change also parameterized range so that we can change it
easily.

Non printable ascii is printed as ".".

This commit also fixes defaltehd always reports output length is 0.

This identifier string is used if HTTP/2 is used over cleartext TCP.

NGHTTP2_CLIENT_CONNECTION_PREFACE has the same content with
NGHTTP2_CLIENT_CONNECTION_HEADER, which is now obsoleted by
NGHTTP2_CLIENT_CONNECTION_PREFACE.

pending_local_max_concurrent_stream is now set in
nghttp2_session_add_settings, rather than after frame was sent.

The number of closed stream to keep is limited by
MAX_CONCURRENT_STREAMS - current active stream.

This is useful because application may allocate resources for it and
wants to free the resources if they are not used anymore.