MINOR: proxy: Add use-small-buffers option to set where to use small buffers

MINOR: proxy: Add use-small-buffers option to set where to use small buffers

Thanks to previous commits, it is possible to use small buffers at different
places: to store the request when a connection is queued or when L7 retries
are enabled, or for health-checks requests. However, there was no
configuration parameter to fine tune small buffer use.

It is now possible, thanks to the proxy option "use-small-buffers".
Documentation was updated accordingly.