Let serializers write to disk

[berland]

This is opposed to have the transmitter open filehandles
for the serializers to write to.

This is needed in order to serialize binary files.

Issue
Resolves #2382

Approach
Move the async opening of files into the serializer encode/decode functions.

[Blunde1]

Sync version of serializer is exposed due to module _record.py also being sync.
Perhaps it makes sense for _record.py to be async?
In that case:

• keep sync version of serializer, to be able to adhere to strangulation pattern and isolating refactoring the serializer
• Write issue on making _record.py async, referencing sync version of serializer that could be removed in the same PR

[jondequinor]

This is gold