Anonymous namespaces are forbidden in headers, as they cause violations of the one-definition rule.

One was added to command.h (SERVER-52542) a while back.
https://mongodbcr.appspot.com/737630008/patch/762060001/754440009

We should take that out and/or replace it with something like a command_details namespace.

I can roll this into other work on the Command.h header for the "shard+router" project.