Restore DisablePEX option functionality

There is a `-disable-pex` (`DisablePEX`) option but it does not do anything except prevent pex from cleaning up expired peers in its peerlist.

What it should do is disable the sending of and the processing of pex messages.

Start by locating the Go implementation of the -disable-pex (DisablePEX) option and the peer-exchange message send and processing paths. Verify the option's behavior with the relevant tests, and consider the work complete when enabling it prevents both sending and processing PEX messages while retaining expected peer-list behavior.

Written by the indexing model from the issue text.