When the parse(IDLParserContext&, BSONObj&) method is called, we automatically assume this is a command request and construct the serialization context accordingly.  However, this method can also be called when building oplog entries, so we need to be able to override the default serialization context being constructed in parse() by passing a non-default one in via the IDLParserContext.

This currently affects importCollection and collMod.  As the goal of these changes are in pursuit of fixing tests that will fail once serializeForCommands and deserializeForCommands are plumbed in, no new tests are being added for this ticket.