Dual DHT doesn't filter QueryEvents

[willscott]

the wrapper for multiple DHTs will result in a union of errors being emitted when a client asks to RegisterForQueryEvents. This means that if either DHT fails, some clients may interpret the overall call as a failure and give up.

The Dual DHT should generate sub-contexts for its WAN and LAN DHTs, and route these events in the same way that it streams and combines other asynchronous output from them.