Admins can now queue multiple messages and send them when they are
ready. This is fully compatible with the previous global behaviour.

Admins can now also send messages to individual servers. This is
useful for informing users of maintenance due to downtime.

Having these throw is terrible for ergonomics and there are loads
of places where the exception was either silently ignored or not
handled at all. Having a function which returns an optional and
another that returns a default works a lot better imo.

Closes #369.

This will be useful for doing challenge authentication on InspIRCd.