Send chunked responses if body size is unknown.

Send chunked responses if body size is unknown.

Apply HTTP chunked transfer encoding to the response body sent to client
if all of the following conditions are met:

* client claims HTTP version 1.1 or later support
* response does not have a Content-Length header already
* response does not use multipart/byteranges encoding
* connection is persistent

If we decide to send chunked reply, chunked_reply flag is set. Chunked
encoding is done in ClientSocketContext::packChunk(). The last-chunk
is sent only when clientReplyContext complete flag is set.

This change helps keep client-side connections persistent.