Enable client-peer blocklists

[bemson]

Clients need a two-way block feature. Similar to blocking a user in an IRC chatroom, clients (i.e., apps) need the means to ignore other clients, when sending and receiving network traffic.

At this point, such a feature would only be enforced at the SubEtha-Client module level. As such, it would not involve additional protocols between the client and bridge.

Finally, this will be done after security enhancements are made to the client.