[ML] Read one object at a time for incremental training state

[tveasey]

Currently, we read one big document for the data summarisation and one big document for the inference model, but there is nothing to stop us reading individual state documents one at a time using the doc.ParseStream<rapidjson::kParseStopWhenDoneFlag> API. This would significantly cutdown the memory when of restoring state for incremental train since we only inflate the state on-the-fly when reading the dechunked stream. Relates to #1849.