Implement parsing of .window and .threshold in backend polling specifications.

Implement parsing of .window and .threshold in backend polling specifications.

.window is how many of the latest polls we examine.
.threshold is how many must have succeeded for the backend to be healthy.

	.window = 40;
	.threshold = 30;
		// Thirty of the last fourty polls must have succeed.

	.threshold = 4;
		// The last four polls must have succeeded.

Default values:
	.window = 8;
	.threshold = 3;

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@3098 d4fa192b-c00b-0410-8231-f00ffab90ce4