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

Commencez par reproduire le problème avec le mot-clé Log de Robot Framework et la commande fournie robot --listener allure_robotframework, en comparant le nombre de pièces jointes pour une petite suite. Suivez le traitement des messages de log par le listener allure_robotframework. Le travail est terminé lorsque les pièces jointes de logging de toute l’exécution sont regroupées dans un seul fichier sans perte de messages, avec le comportement vérifié sur une suite plus grande.

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