Add support for multiple DBClients

[aasheptunov]

Add support for multiple DBClients.

User story:
As a developer, I want to modify the way DBClients works, so that it supports multiple database connections.

DoD:

• The API can establish multiple connections to the database (or databases).

[aasheptunov]

Before performing the task and modifying the DBClients logic, you must design and implement load testing for the API so that you can compare the results before and after, as well as with the .NET API.

Task #65.