Server-specified event filtering

[jdswinbank]

Currently, it's possible for clients to request that the server perform some filtering on their behalf.

However, it would be nice for the server admin to be able to impose some blanket filtering, i.e. "do not forward any events matching pattern X to any subscriber".

This would be easy to add to the EventRelay handler. Perhaps EventRelay should just become a plugin rather than a core part of the broker.