Buffer request from a single user

**[Original report](https://bitbucket.org/hzgwpn/jdataformatserver/issue/4) by Ingvord (Bitbucket: [Ingvord](https://bitbucket.org/Ingvord), GitHub: [Ingvord](https://github.com/Ingvord)).**

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Accumulate several request from a single user into a buffer and then submit it to executor.

TransactionMode can be switched to NONE after this task
