Add Serialized wrapper to make it harder to accidentally deserialize the wrong type

[Kobzol]

While restoring the dashboard, I realized that the comment about the type of the Submit::serialized_desc was wrong - it was not storing JobDescription, but SubmitRequest. There was also one place in the code that was deserializing this incorrect type. I created a type wrapper to make sure that this doesn't happen again.

We should use the same thing for SerializedTaskContext, however it currently uses different bincode options (it uses fixint encoding and doesn't allow trailing bytes), so I did not use it there yet.

[spirali]

Nice idea.

Both serialization of SubmitRequest and SerializedTaskContext should not allow trailing spaces. I think that it is ok to synchronize both on the same serializer. My first guess would be variable encoding but I did do any benchmarking so also fixint is probably ok.