Reorganized account registration notifications

Ignore-this: 680c38b27f632169bd2466a5132bc56d

darcs-hash:20100106144724-1e3d8-5cc5a17c8353a3e80c6e176194b9930e318dc989
Improved test for race conditions in AccountRegistrar

Ignore-this: eff6fc234bd28f501296c2e929c1eb1d

darcs-hash:20100106155931-1e3d8-7b40a86f9df7b4576b8b352fd0e33edc07a0b621