[MAJOR] buffer: flag BF_DONT_READ to disable reads when not required

[MAJOR] buffer: flag BF_DONT_READ to disable reads when not required

When processing a GET or HEAD request in close mode, we know we don't
need to read anything anymore on the socket, so we can disable it.
Doing this can save up to 40% of the recv calls, and half of the
epoll_ctl calls.

For this we need a buffer flag indicating that we're not interesting in
reading anymore. Right now, this flag also disables both polled reads.
We might benefit from disabling only speculative reads, but we will need
at least this flag when we want to support keepalive anyway.

Currently we don't disable the flag on completion, but it does not
matter as we close ASAP when performing the shutw().