Make `StdKeySerializers` use new `JsonGenerator.writeFieldId()` for `int`/`long` keys

[cowtowncoder]

With addition of JsonGenerator.writeFieldId() in JsonGenerator in 2.8, it will be possibly to support int/long valued keys for data formats like CBOR. But due to timing of addition changes for databind won't make it in 2.8, so let's target this for 2.9.

[cowtowncoder]

I think this will require addition of separate Integer/Long key serializers as the first step (otherwise will call value.toString() to lose value). Since fieldId is of type long, probably should just use one that accepts Number, to call longValue() on.