message_service: add sanity check that rpc connections are not created in the maintenance mode

In maintenance mode, a node shouldn't be able to communicate with other nodes.

To make sure this does not happen, the sanity check is added.

main.cc

@@ -1246,6 +1246,7 @@ To start the scylla server proper, simply invoke as: scylla server (or just scyl
             if (snitch.local()->prefer_local()) {
                 mscfg.preferred_ips = sys_ks.local().get_preferred_ips().get0();
             }
+            mscfg.maintenance_mode = maintenance_mode_enabled{cfg->maintenance_mode()};
 
             const auto& seo = cfg->server_encryption_options();
             auto encrypt = utils::get_or_default(seo, "internode_encryption", "none");

message/messaging_service.cc

@@ -802,6 +802,9 @@ gms::inet_address messaging_service::get_public_endpoint_for(const gms::inet_add
 
 shared_ptr<messaging_service::rpc_protocol_client_wrapper> messaging_service::get_rpc_client(messaging_verb verb, msg_addr id) {
     assert(!_shutting_down);
+    if (_cfg.maintenance_mode) {
+        on_internal_error(mlogger, "This node is in maintenance mode, it shouldn't contact other nodes");
+    }
     auto idx = get_rpc_client_idx(verb);
     auto it = _clients[idx].find(id);
 

message/messaging_service.hh

@@ -8,6 +8,7 @@
 
 #pragma once
 
+#include "db/config.hh"
 #include "messaging_service_fwd.hh"
 #include "msg_addr.hh"
 #include <seastar/core/seastar.hh>
@@ -22,6 +23,7 @@
 #include "streaming/stream_fwd.hh"
 #include "locator/host_id.hh"
 #include "service/session.hh"
+#include "service/maintenance_mode.hh"
 
 #include <list>
 #include <vector>
@@ -281,6 +283,7 @@ public:
         bool listen_on_broadcast_address = false;
         size_t rpc_memory_limit = 1'000'000;
         std::unordered_map<gms::inet_address, gms::inet_address> preferred_ips;
+        maintenance_mode_enabled maintenance_mode = maintenance_mode_enabled::no;
     };
 
     struct scheduling_config {