Reduce memory footprint of state messages

[laurentS]

PR copied from oviohub#2

This PR helps substantially reduce the memory footprint of the target by storing the singer STATE message in their raw string format instead of the deserialized objects.

The downside is that STATE messages are deserialized twice instead of just one time, so there is some slight performance penalty. However, this only really applies to 1/N such messages, with N depending on the max__batch_rows config setting, but typically appearing to be >100.

In practice, we've seen a memory footprint reduction by a factor of 15-20x with this change, this will of course depend on how your STATE messages are structured.