code: public void deleteConversation(@NonNull final String conversationId, final String eTag, @Nullable Callback<ComapiResult<Void>> callback) {
          adapter.adapt(deleteConversation(conversationId, eTag), callback);
      }
docs: Returns observable to create a conversation.
  
  @param conversationId ID of a conversation to delete.
  @param eTag           ETag for server to check if local version of the data is the same as the one the server side.
  @param callback       Callback to deliver new session instance.