Buffers reuse in chunked filter.

Buffers reuse in chunked filter.

There were 2 buffers allocated on each buffer chain sent through chunked
filter (one buffer for chunk size, another one for trailing CRLF, about
120 bytes in total on 32-bit platforms).  This resulted in large memory
consumption with long-lived requests sending many buffer chains.  Usual
example of problematic scenario is streaming though proxy with
proxy_buffering set to off.

Introduced buffers reuse reduces memory consumption in the above problematic
scenario.

See here for initial report:
http://mailman.nginx.org/pipermail/nginx/2010-April/019814.html