Use descriptor name instead of class name

res_testing.OrderCreated instead of ResTesting::OrderCreated,
this is less ruby-related and more protobuf oriented. If that descriptor
name is used throught many apps, it should be easier to handle it
compared to ruby classes.

Issue: #228

rails_event_store/spec/protobuf_integration_spec.rb

@@ -7,7 +7,7 @@ module RailsEventStore
       client = Client.new(repository: RailsEventStoreActiveRecord::EventRepository.new(
         mapper: RubyEventStore::Mappers::Protobuf.new,
       ))
-      client.subscribe(->(ev){@ev = ev}, [ResTesting::OrderCreated])
+      client.subscribe(->(ev){@ev = ev}, [ResTesting::OrderCreated.descriptor.name])
       event = RubyEventStore::Proto.new(
         data: ResTesting::OrderCreated.new(
           customer_id: 123,

ruby_event_store/lib/ruby_event_store/mappers/protobuf.rb

@@ -7,7 +7,7 @@ def initialize(event_id: SecureRandom.uuid, metadata: nil, data: nil)
     end
 
     def type
-      data.class.name
+      data.class.descriptor.name
     end
   end
 
@@ -29,7 +29,7 @@ def event_to_serialized_record(domain_event)
 
       def serialized_record_to_event(record)
         event_type = events_class_remapping.fetch(record.event_type) { record.event_type }
-        data = Object.const_get(event_type).decode(record.data)
+        data = Google::Protobuf::DescriptorPool.generated_pool.lookup(event_type).msgclass.decode(record.data)
         Proto.new(
           event_id: record.event_id,
           data: data,