IF: HS network message filtering infrastructure (net plugin <-> IF engine)

[fcecin]

Add code for:

• Net plugin notifies the connection_id of incoming HS messages to IF engine (this will be done by an additional parameter in the existing callback function signature(s)).

• IF engine can (optionally) supply a connection_id to exclude, when passing an outgoing (broadcast) HS message to Net plugin; this is done whenever the IF engine will (in the future) decide to propagate a message, instead of sending an original message to broadcast.

• IF engine calls the Net plugin whenever it detects that it has received a potentially malicious message (duplicates, irrelevant messages, messages with invalid signatures, i.e. an enum "reason"...) from a given connection_id direct peer, allowing the Net plugin to disconnect that direct peer that it decides is misbehaving (for example, by counting how frequently each type of messaging offense happen in a counter that would be added to the corresponding connection object -- which could then e.g. "cool down" periodically, or maybe not).