Export httpclient_create_proxy() in http_client.h

httpclient_new_from_proxy() is a variant of httpclient_new() which
allows to create the requests from a different proxy.

The proxy and its 2 servers are now stored in the httpclient structure.

The proxy must have been created with httpclient_create_proxy() to be
used.

The httpclient_postcheck() callback will finish the initialization of
all proxies created with PR_CAP_HTTPCLIENT.

Add the ability to set a "server timeout" on the httpclient with either
the httpclient_set_timeout() API or the timeout argument in a request.

Issue #1470.

httpclient_set_dst() allows to set an alternative destination address
using HAProxy addres format. This will ignore the address within the
URL.

This patch add a way to handle HTTP requests streaming using a
callback.

The end of the data must be specified by using the "end" parameter in
httpclient_req_xfer().

httpclient_req_gen() takes a payload argument which can be use to put a
payload in the request. This payload can only fit a request buffer.

This payload can also be specified by the "body" named parameter within
the lua. httpclient.

It is also used within the CLI httpclient when specified as a CLI
payload with "<<".

During httpclient_destroy, add a condition in the BUG_ON which checks
that the client was started before it has ended. A httpclient structure
could have been created without being started.

httpclient_stop_and_destroy() tries to destroy the httpclient structure
if the client was stopped.

In the case the client wasn't stopped, it ask the client to stop itself
and to destroy the httpclient structure itself during the release of the
applet.

httpclient_ended() returns 1 if there is no more data to collect,
because the client received everything or the connection ended.

httpclient_data() returns the available data in the httpclient.