Remove presence notifications

[futurechimp]

We should remove presence notifications from all Rust nodes, we can now track things through node announcements in the blockchain.

[futurechimp]

I'll also include removing presence from directory stuff in the validators here. When this is done, all presence notifications will be gone from the full system.

[futurechimp]

This is now gone from validators and the metrics server. Still need to remove all presence-related stuff from the mixnet nodes.