[web] decrypt and persist plaintext in SQL transaction

Summary:
This code attempt to decrypt and on success in one transaction persist plaintext and crypto module state.

Depends on D11727

Test Plan: Decrypt some messages and then query DB to make sure content is persisted.

Reviewers: tomek, marcin

Reviewed By: marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11728