we should ensure implementations of all shims in output binaries

The MONGO_SHIM macros don't actually verify (at compile, link or startup) that they have implementations. We can't enforce this in the initializer graph (because the point of shim is to let us have unimplemented symbols for unittests), but we should check it for binaries intended for end user consumption.

This would probably take the form of a initializer than ran after all shims, or an explicit call to some static function that checked against a global registry.