Deduplicate multiply defined member functions of the mongo::Command class

The Command class methods Command::registerError, Command::findCommand, and Command::execCommand are currently implemented differently in mongod and mongos. This means that any library that depends on the Command class cannot fully express its dependencies, since it cannot know whether it is ultimately to be linked into mongod or mongos. We should either find a way to unify the implementations, or find a way to dependency inject different helper implementations.