BUG/MINOR: channel: initialize xfer_small/xfer_large on new buffers

These ones are only reset during transfers. There is a low but non-null
risk that a first full read causes the previous value to be reused and
immediately to immediately set the CF_STREAMER flag. The impact is only
to increase earlier than expected the SSL record size and to use splice().

This bug was already present in 1.4, so a backport is possible.
MINOR: channel: add the date of last read in the channel

We store the time stamp of last read in the channel in order to
be able to measure some bit rate and pause lengths. We only use
16 bits which were unused for this. We don't need more, as it
allows us to measure with a millisecond precision for up to 65s.
MEDIUM: stream-int: automatically disable CF_STREAMER flags after idle

Disabling the streamer flags after an idle period will help TCP proxies
to better adapt to the streams they're forwarding, especially with SSL
where this will allow the SSL sender to use smaller records. This is
typically used to optimally relay HTTP and derivatives such as SPDY or
HTTP/2 in pure TCP mode when haproxy is used as an SSL offloader.

This idea was first proposed by Ilya Grigorik on the haproxy mailing
list, and his tests seem to confirm the improvement :

  https://www.mail-archive.com/haproxy@formilux.org/msg12576.html
MINOR: ssl: add DEFAULT_SSL_MAX_RECORD to set the record size at build time

For some deployments it may help to have tune.ssl.maxrecord set to a
more efficient one at build time. This build setting allows this.
MINOR: config: make the stream interface idle timer user-configurable

The new tune.idletimer value allows one to set a different value for
idle stream detection. The default value remains set to one second.
It is possible to disable it using zero, and to change the default
value at build time using DEFAULT_IDLE_TIMER.