Delay the deprecation warning of use_client_provided_uniq_id (#26)

When setting the `use_client_provided_uniq_id` to false through Ruby
configuration code the deprecation warning was still issued:

```ruby
GraphQL::AnyCable.config.use_client_provided_uniq_id = false

# or...

GraphQL::AnyCable.configure do |config|
  config.use_client_provided_uniq_id = false
end
```

This happened because the deprecation warning was issued through an
`on_load` callback that is invoked when the `GraphQL::AnyCable.config`
is initialised. At that time, Anyway is getting settings through sources
like ENV variables, but the Ruby configuration code is invoked
**afterwards**. But we prefer having this setting in Ruby code, not
in an ENV or a YAML setting file.

lib/graphql/anycable/config.rb

@@ -12,14 +12,6 @@ class Config < Anyway::Config
       attr_config use_redis_object_on_cleanup: true
       attr_config handle_legacy_subscriptions: false
       attr_config use_client_provided_uniq_id: true
-
-      on_load do
-        next unless use_client_provided_uniq_id?
-
-        warn "[Deprecated] Using client provided channel uniq IDs could lead to unexpected behaviour, " \
-             " please, set GraphQL::AnyCable.config.use_client_provided_uniq_id = false or GRAPHQL_ANYCABLE_USE_CLIENT_PROVIDED_UNIQ_ID=false, " \
-             " and update the `#unsubscribed` callback code according to the latest docs."
-      end
     end
   end
 end

lib/graphql/anycable/railtie.rb

@@ -9,6 +9,14 @@ class Railtie < ::Rails::Railtie
         path = File.expand_path(__dir__)
         Dir.glob("#{path}/tasks/**/*.rake").each { |f| load f }
       end
+
+      config.after_initialize do
+        if GraphQL::AnyCable.config.use_client_provided_uniq_id?
+          warn "[Deprecated] Using client provided channel uniq IDs could lead to unexpected behaviour, " \
+               "please, set GraphQL::AnyCable.config.use_client_provided_uniq_id = false or GRAPHQL_ANYCABLE_USE_CLIENT_PROVIDED_UNIQ_ID=false, " \
+               "and update the `#unsubscribed` callback code according to the latest docs."
+        end
+      end
     end
   end
 end