* cleanup: remove UnderlyingNetworkLibrary and TProxy
While there, replace mixture of mocking and real connections inside
quicping with pure mocking of network connections.
Closes https://github.com/ooni/probe/issues/2224
* cleanup: we don't need a SimpleResolver now
This type was only used by UnderlyingNetworkLibrary and all the
rest of the code uses Resolver. So, let's avoid complexity by zapping
the SimpleResolver type and merging it inside Resolver.