replication: make anon replicas iteration safe

In replicaset_follow we iterate anon replicas list: list of replicas
that haven't received an UUID. In case of successful connect replica
link is being removed from anon list. If it happens immediately,
without yield in applier, iteration breaks. Now it is fixed by
rlist_foreach_entry_safe instead of common rlist_foreach_entry.
Relevant test case is added.

Part of #4586
Closes #4576
Closes #4440

src/box/replication.cc

@@ -802,7 +802,8 @@ replicaset_follow(void)
 		if (replica->applier != NULL)
 			applier_resume(replica->applier);
 	}
-	rlist_foreach_entry(replica, &replicaset.anon, in_anon) {
+	struct replica *tmp;
+	rlist_foreach_entry_safe(replica, &replicaset.anon, in_anon, tmp) {
 		/* Restart appliers that failed to connect. */
 		applier_start(replica->applier);
 	}

test/replication/box_set_replication_stress.result

+-- test-run result file version 2
+test_run = require('test_run').new()
+ | ---
+ | ...
+SERVERS = {'master_quorum1', 'master_quorum2'}
+ | ---
+ | ...
+
+-- Deploy a cluster.
+test_run:create_cluster(SERVERS)
+ | ---
+ | ...
+test_run:wait_fullmesh(SERVERS)
+ | ---
+ | ...
+
+test_run:cmd("switch master_quorum1")
+ | ---
+ | - true
+ | ...
+repl = box.cfg.replication
+ | ---
+ | ...
+for i = 1, 1000 do              \
+    box.cfg{replication = ""}   \
+    box.cfg{replication = repl} \
+end
+ | ---
+ | ...
+test_run:cmd("switch default")
+ | ---
+ | - true
+ | ...
+
+-- Cleanup.
+test_run:drop_cluster(SERVERS)
+ | ---
+ | ...

test/replication/box_set_replication_stress.test.lua

+test_run = require('test_run').new()
+SERVERS = {'master_quorum1', 'master_quorum2'}
+
+-- Deploy a cluster.
+test_run:create_cluster(SERVERS)
+test_run:wait_fullmesh(SERVERS)
+
+test_run:cmd("switch master_quorum1")
+repl = box.cfg.replication
+for i = 1, 1000 do              \
+    box.cfg{replication = ""}   \
+    box.cfg{replication = repl} \
+end
+test_run:cmd("switch default")
+
+-- Cleanup.
+test_run:drop_cluster(SERVERS)