Context

Currently the driver allows marshaling the WriteConcern struct but doesn’t convert the Journal struct member to `j` … which can produce an invalid `writeConcern` BSON document.

This is important because users who call RunCommand() are fairly likely to do something like:

RunCommand(
    ctx,
    bson.D{
        update: "mycoll",
        updates: []bson.D{ … },
        writeConcern: writeconcern.Majority(),
    },
)

… which right now happens to work, but if there’s anything in the WriteConcern.Journal field then an invalid BSON document will result. (I believe WTimeout is also improperly marshaled since it’s a Duration, and the expected value is a # of milliseconds.)

I believe something similar may affect ReadConcern. There, though, per matt.dale@mongodb.com it’s actually not possible to marshal the document reliably/completely. In that case, mayhap an error-returning MarshalBSON() method would be indicated?

Definition of done

Marshaling a WriteConcern or a ReadConcern should return either an error or a correct result, not an incorrect document.