allure-robotframework creates separate attachment files for log messages from each keyword

#### I'm submitting a ...
- [***] bug-feature report

#### What is the current behavior?
Currently allure-robotframework creates separate attachment files for log messages from each keyword, no matter if it is from external library or from builtin Log keyword.

#### If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem
Unfortunately can't provide any test results. I think, that it will be enough to create few tests with Log keyword to see result.

To launch test project i use this command line:
robot --listener allure_robotframework --loglevel DEBUG -T --outputdir .\log\robot --variable REMOTE_URI:127.0.0.1:1234 --console verbose -s 'List_Test' .\x-tests

#### What is the expected behavior?
I expect that all **logging** attachments for whole test project run should go in to one attachment file.

#### What is the motivation / use case for changing the behavior?
This behavior can inflict problem with accessing output folder when running complex test suites with many log messages. For example, simple test suite with two normal tests generates 84 attachment files. You may imagine what will happens with output directory when a really big test project will run few times.

#### Please tell us about your environment:

- Allure version: 2.10.0
- Test framework: Robot Framework 3.1 (Python 3.6.7 on win32)
- Allure adaptor: allure-robotframework@2.6.0

まず、Robot Framework の Log キーワードと、提供された robot --listener allure_robotframework コマンドを使って問題を再現し、小規模なスイートの添付ファイル数を比較します。allure_robotframework リスナーのログメッセージ処理を追跡します。完了条件は、メッセージを失うことなく実行全体のロギング添付ファイルが 1 つのファイルに統合され、より大規模なスイートで動作が検証されることです。

索引モデルが issue の本文から書いたものです。