Web-server: send large resources in chunked mode

[krizhanovsky]

After #471 implementation we need to be able to send large resources in chunked encoding instead of Content-Length only. It seems the chunks length should be near to page size to perfectly work with TDB data and skb fragments.

#391 first points are related to the issue. Issue #498 requires proxy_buffering option which is related to the issue.

[krizhanovsky]

Also consider #638 and corresponding issue #639 (comment) : we must be able to transform response body to chunked encoding.

[krizhanovsky]

This is HTTP/1 specific and the protocol isn't crucial for the web server mode anymore