[core] Allow setting attributes to the bot via WillieWrapper

Most people don't realise the wrapper exists, so they think they could
modify bot attributes from module context. Until now, they couldn't.

The startup routine in coretasks set bot.connection_registered. Without this
commit, setting bot attributes in module context would only set them on
the ephemeral wrapper. This behaviour caused startup() in coretasks to repeat
twice, once for 001 RPL_WELCOME and once for 251 RPL_LUSERCLIENT.
With this commit, startup() is only called once.