Write integer to log buffer directly to improve efficiency.  Remove
unused function templates.  Use [first, last) style arguments for
copy() function templates.

Previously, when parsing HTML links, we only take into account
overridden host.  But we actually need more variables to consider.  In
this commit, we take into account overridden scheme, host, and port to
parse HTML links.

Fix authority for --get-assets if IP adress is used in conjunction with user-defined :authority header

--accesslog-write-early option is analogous to HAProxy's logasap.  If
used, nghttpx writes access log when response header fields are
received from backend rather than when request transaction finishes.

access.log timestamp is now when request header fields are received,
rather than when access log is written.

nghttpx: Use Memchunk based read buffer for frontend connection

Previously, we have dedicated read buffer for each frontend
connection.  With this commit, the buffer spaces are only used when
needed, and pooled if they are not used.  This reduces memory usage
for idle client connections.