This adds support for naming the source iterators of FairMix, like so:
mix.AddSource(enode.WithSourceName("mySource", iter))
The source that produced the latest node is returned by the new NodeSource method.
In p2p/dial.go, conn.flags was accessed without using sync/atomic.
This race is fixed by removing the access.
In p2p/enode/iter_test.go, a similar race is resolved by writing the field atomically.
Co-authored-by: Felix Lange <fjl@twurst.com>