Store `clientDataSetId` on a StorageContext instance as a private

[rvagg]

This is the internal nonce that's needed for createDataSet (changing to be random with FilOzone/filecoin-services#265 but currently sequential). There are 2 places where we need this afterwards:

1. AddPieces - we get it in getAddPiecesInfo where it's one of 4 parallel calls. I think that's all we use from viewContract.getDataSet so this would reduce that by one call.
2. DeletePiece - in feat: add piece deletion to storage context #285, where we need to make a dedicated call to get it before we can delete.

I don't know why we use it for these two and not just the dataSetId, but assuming that's not changing, we should just keep it when we resolve the provider and data set on create().

When #264 becomes a thing, this is just like dataSetId in that it's lazily resolved and until then it's an error to try and use it.