This fixes a race condition in the onSocket listener method.
onSocket gets a net.Socket parameter that needs to be closed later
before the tcp server can close.
The net.Socket is used to create a MultiaddrConnection but the
connection is not tracked with this.connections until after it has been
upgraded.
If the tcp listener.close is called while a the MultiaddrConnection is waiting
to be upgraded, then the MultiaddrConnection socket cannot be closed as
it does not exist in this.connections.

Instead of adding the MultiaddrConnection to this.connections on
creation I decided to create a separate property named
this.maConnections. I decided to track the non-upgraded connections
separately because I was unsure how this.connections must be used with other things
like metrics.