Use writeStartObject(Object forValue, int size) for ObjectNode serialization

[cowtowncoder]

(note: part of #3679)

Currently ObjectNode serialization uses "simple" writeStartObject(Object forValue).
It would be more efficient for some backends to use variant that takes number of elements to write as argument (f.ex CBOR has length-prefixed variant that may be more efficient to decode).

Although not all cases can use this variant (if filtering, f.ex), but some can: let's use it where we can.