Adding a test 'Concurrency 3'.
Adding a test 'Concurrency 3'.

The test runs following actions.
- db1: insert 1000 rows
- db2: make 100 transactions and each one insert 10 rows
- db3: make 10 transactions ane each one insert 100 rows

The test expect all above actions are scheduled correctly between 2 threads without breaking DatabaseWorkerPool's rules.

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