Lock subs when reading it

should probably make a copy instead of doing io in the mutex

src/server/main.zig

@@ -202,6 +202,8 @@ fn publishMessage(server: *Server, io: std.Io, alloc: std.mem.Allocator, source_
         }
     }
     defer msg.deinit(alloc);
+    try server.subs_lock.lock(io);
+    defer server.subs_lock.unlock(io);
     for (server.subscriptions.items) |subscription| {
         if (subjectMatches(subscription.subject, msg.subject)) {
             const client = server.clients.get(subscription.client_id) orelse {