The MongoProcessInterface class hierarchy has grown significantly over time, and is now quite unwieldy. The names of the classes and files are misleading and inconsistent (e.g. we have process_interface_standalone, process_interface_shardsvr and mongos_process_interface but no process_interface_repl_node), with much explicitly sharding-related code incongruously located in MongoInterfaceStandalone and some methods which have fundamental assumptions about their callers baked in, which can lead to subtle bugs (see e.g. SERVER-44977).

We should refactor this into a more sensible and generic hierarchy that any code can confidently call.