When working on
MDL-16879 and testing the upgrade_fix_incorrect_mnethostids() function, I've detected that the "all hosts" record of mnet_hosts isn't created anymore by lib/db/install.php. Only the "localhost" one is created.
Note that mnet_enviroment->init() continues having it and also the upgrade_fix_incorrect_mnethostids() function recreates it if missing.
So one of these must be achieved:
1) if this is really on bug in lib/db/install.php ---> it should be fixed there, by adding the "all hosts" record.
2) if that record isn't needed any more, both mnet_enviroment->init() and upgrade_fix_incorrect_mnethostids() must delete any use.
Note I think it's 1), because it seems that there are a bunch of uses of "mnet_all_hosts_id" here and there in core.