swim: make members array decoder be a separate function

At this moment SWIM protocol stores array of members only in one
place: inside the anti-entropy component. Its decoding is a
simple loop taking the member definitions one by one and
upserting them into the member table.

But the dissemination also has something kinda like members
array: an array of events. The trick is that an event is
basically the same as a member +/- a couple of optional fields.
Events are also decoded into the member definition structure. It
means that anti-entropy decoder can be easily reused.

Part of #3234

src/lib/swim/swim.c

@@ -1019,12 +1019,14 @@ swim_upsert_member(struct swim *swim, const struct swim_member_def *def,
 	return 0;
 }
 
-/** Decode an anti-entropy message, update member table. */
+/**
+ * Decode a bunch of members encoded as a MessagePack array. Each
+ * correctly decoded member is upserted into the member table.
+ */
 static int
-swim_process_anti_entropy(struct swim *swim, const char **pos, const char *end)
+swim_process_members(struct swim *swim, const char *prefix,
+		     const char **pos, const char *end)
 {
-	say_verbose("SWIM %d: process anti-entropy", swim_fd(swim));
-	const char *prefix = "invalid anti-entropy message:";
 	uint32_t size;
 	if (swim_decode_array(pos, end, &size, prefix, "root") != 0)
 		return -1;
@@ -1044,6 +1046,15 @@ swim_process_anti_entropy(struct swim *swim, const char **pos, const char *end)
 	return 0;
 }
 
+/** Decode an anti-entropy message, update member table. */
+static int
+swim_process_anti_entropy(struct swim *swim, const char **pos, const char *end)
+{
+	say_verbose("SWIM %d: process anti-entropy", swim_fd(swim));
+	const char *prefix = "invalid anti-entropy message:";
+	return swim_process_members(swim, prefix, pos, end);
+}
+
 /**
  * Decode a failure detection message. Schedule acks, process
  * acks.