MongoDB: add support of transactions

[DenisRazinkin]

For scenarios that require atomicity of read and write operations across multiple documents (within a single collection or multiple collections), MongoDB supports distributed transactions.
Transactions are helpful where partial execution across multiple collections can result in non-consistent data condition.

[ArkadyRudenko]

Yes, transactions are useful but there is no such goal in our plans for the near future. Maybe we will try to impl it in the end of Q3 but I don`t promise

PRs are welcome :)