[MAJOR] proxy: finally get rid of maintain_proxies()

This function is finally not needed anymore, as it has been replaced with
a per-proxy task that is scheduled when some limits are encountered on
incoming connections or when the process is stopping. The savings should
be noticeable on configs with a large number of proxies. The most important
point is that the rate limiting is now enforced in a clean and solid way.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index e5d49ed..82c3458 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -306,6 +306,7 @@
 	struct list listener_queue;		/* list of the temporarily limited listeners because of lack of a proxy resource */
 	struct stktable table;			/* table for storing sticking sessions */
 
+	struct task *task;			/* the associated task, mandatory to manage rate limiting, stopping and resource shortage */
 	int grace;				/* grace time after stop request */
 	char *check_req;			/* HTTP or SSL request to use for PR_O_HTTP_CHK|PR_O_SSL3_CHK */
 	int check_len;				/* Length of the HTTP or SSL3 request */