TODO: We should decide whether we want channel descriptors to be housed within each reactor (as they are now) or, considering that the reactor doesn't really need to care about the channel descriptors, if they should be housed in the node module.

NewReactor returns a reference to a new reactor.

PexChannel is a channel for PEX messages.

The peer exchange or PEX reactor supports the peer manager by sending requests to other peers for addresses that can be given to the peer manager and at the same time advertises addresses to peers that need more.