Query Mode filtering

[cheme]

Query mode are currently always accessible, this leads to serious issue (totally unsecure), replying in 'proxy' mode to stuck a peer or sending 'aproxy' query in some sensitive network, and a lot of possible misuse.

Possible query mode should be configurable. We need a rules to return the allowed mode for a given dht (a simple bit mask), with obvious filtering on message reception in server.

More advanced filtering should also be based upon kind of query, so an optional parameter with &key could be added. This will be usefull with network where multiple mode are used for multiple usecases.