MINOR: server: increase the default pool-purge-delay to 5 seconds

The default used to be a very aggressive delay of 1 second before starting
to purge idle connections, but tests show that with bursty traffic it's a
bit short. Let's increase this to 5 seconds.
diff --git a/src/server.c b/src/server.c
index 541dc69..97cc5e0 100644
--- a/src/server.c
+++ b/src/server.c
@@ -1811,7 +1811,7 @@
 	srv->agent.proxy = proxy;
 	srv->xprt  = srv->check.xprt = srv->agent.xprt = xprt_get(XPRT_RAW);
 
-	srv->pool_purge_delay = 1000;
+	srv->pool_purge_delay = 5000;
 	srv->max_idle_conns = -1;
 	srv->max_reuse = -1;