Added support for load balancing among backends in varnish. It is still experimental

Added support for load balancing among backends in varnish. It is still experimental
and very basic, but it should be ready for testing. Two strategies for load balancing
are implemented: a simple round robin, and a simple weighted random. The following
is an example configuration in vcl. The weight parameter for random is optional. Default
is equal weight.

backend foo {
	set backend.host = "foo.bar.com";
	set backend.port = "http";
}

backend_round_robin rr {
	set backend.set = {
		{ "foo1.bar.com", "http" }
		{ "foo2.bar.com", "http" }
		{ "foo3.bar.com", "http" }
	};
}

backend_random rrr {
	set backend.set = {
		{ "foo1.bar.com", "http", 0.3 }
		{ "foo2.bar.com", "http", 0.6 }
		{ "foo3.bar.com", "http", 0.1 }
	};
}

sub vcl_recv {
	if {req.http.host ~ "foo"} {
		req.backend = foo;
	} elseif {req.http.host ~ "bar"} {
		req.backend = rr;
	} else {
		req.backend = rrr;
	}
}

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1931 d4fa192b-c00b-0410-8231-f00ffab90ce4