MINOR: server: mark referenced servers as non purgeable

Mark servers that are referenced by configuration elements as non
purgeable. This includes the following list :
- tracked servers
- servers referenced in a use-server rule
- servers referenced in a sample fetch

src/cfgparse.c

@@ -2841,6 +2841,7 @@ int check_config_validity()
 			}
 			ha_free(&srule->srv.name);
 			srule->srv.ptr = target;
+			target->flags |= SRV_F_NON_PURGEABLE;
 		}
 
 		/* find the target table for 'stick' rules */

src/sample.c

@@ -1214,6 +1214,8 @@ int smp_resolve_args(struct proxy *p, char **err)
 				break;
 			}
 
+			srv->flags |= SRV_F_NON_PURGEABLE;
+
 			chunk_destroy(&arg->data.str);
 			arg->unresolved = 0;
 			arg->data.srv = srv;

src/server.c

@@ -4928,6 +4928,7 @@ int srv_apply_track(struct server *srv, struct proxy *curproxy)
 	srv->track = strack;
 	srv->tracknext = strack->trackers;
 	strack->trackers = srv;
+	strack->flags |= SRV_F_NON_PURGEABLE;
 
 	ha_free(&srv->trackit);