In order to give *GlobalParams proper APIs, we need a mechanism for the
MONGO_INITIALIZER* family of macros to give corresponding code access to mutable GlobalParam data structures that the rest of the codebase cannot use.
 
Without some sort of mechanism to bind arbitrary data structures to the InitializerContext at runtime, we'd need to directly add data members to InitializerContext. Doing that would bring all sorts of undesirable dependencies into the InitializerContext, which is used for all executables.