Server-specified IVORN-prefix-based whitelist

[timstaley]

[Wishlist item]
Something of a corollary to #32:
(Until such time as we're all submitting VOEvents with GPG fingerprints, and everyone is verifying packets received according to said fingerprints:)
It would be very handy to apply an ivorn-prefix matching component to remote-ip whitelisting, so e.g.
'Only relay events from remote broker @ 123.4.5.6 if their ivorn begins with the prefix ``ivo://foo.bar.acme/```
This would be a simple way of ensuring that remote brokers do not 'pollute' someone else's stream, either through human error or malicious intervention.

I can think of ways to implement this currently using multiple instances of Comet running on one machine (different ports), by applying filtering within the known-good local instances, but it gets a bit baroque.