rabbit_peer_discovery: Register node before running discovery

[Why]
The two backends that use registration are Consul and etcd. The
discovery process relies on the registered nodes: they return whatever
was previously registered.

With the new checks and failsafes added in peer discovery in RabbitMQ
3.13.0, the fact that registration happens after running discovery
breaks Consul and etcd backend.

It used to work before because the first node would eventually time out
waiting for a non-empty list of nodes from the backend and proceed as a
standalone node, registering itself on the way. Following nodes would
then discover that first node.

Among the new checks, the node running discovery expects to find itself
in the list of discovered nodes. Because it didn't register yet, it will
never find itself.

[How]
The solution is to register first, then run discovery. The node should
at least get itself in the discovered nodes.

(cherry picked from commit cb9f0d8)

Author: dumbbell

deps/rabbit/src/rabbit_db.erl

@@ -52,6 +52,7 @@ init() ->
 
     ensure_dir_exists(),
     rabbit_peer_discovery:maybe_init(),
+    rabbit_peer_discovery:maybe_register(),
 
     pre_init(IsVirgin),
 
@@ -77,7 +78,6 @@ init() ->
                "DB: initialization successeful",
                #{domain => ?RMQLOG_DOMAIN_DB}),
 
-            rabbit_peer_discovery:maybe_register(),
             init_finished(),
 
             ok;

deps/rabbit_common/src/rabbit_peer_discovery_backend.erl

@@ -57,3 +57,8 @@
 -callback unlock(Data :: term()) -> ok.
 
 -optional_callbacks([init/0]).
+
+-export([api_version/0]).
+
+api_version() ->
+    2.