Adjust active work after adding DP dummies

[akoshelev]

When adding dummies to small inputs, we significantly increase the total number of records processed by the protocol, but we never adjust the active work. If we add 600 dummies to 5 row input, our active work would be still 5.

It may not be trivial as we somehow need to replace the context's active work, likely similar to narrow

[akoshelev]

taken from #1300 as a separate issue