refactor: add sendContainer to unify duplicated code

The same logic for constructing and sending an MTPTxContainer was
repeated in dozens of places.
fix some problems uncovered by the refactoring
[mtp] Send OperationNotSupported responses where needed

Some not-implemented operations weren't sending anything back,
which caused a timeout and protocol reset when they were used.
They should return an OperationNotSupported response instead.
[mtp protocol] Implement GetPartialObject request
[tests] Add unit test for use of thumbnailer