race condition in control service

[fosterseth]

@shanemcd discovered this, here are the stack traces

WARNING: DATA RACE
Write at 0x00c00134b4b0 by goroutine 1564:
  runtime.closechan()
      /usr/lib/golang/src/runtime/chan.go:356 +0x0
  github.com/ansible/receptor/pkg/netceptor.(*PacketConn).Close()
      /home/shanemcd/receptor/pkg/netceptor/packetconn.go:207 +0x1ea
  github.com/ansible/receptor/pkg/netceptor.(*Listener).Close()
      /home/shanemcd/receptor/pkg/netceptor/conn.go:260 +0x84
  github.com/ansible/receptor/pkg/controlsvc.(*Server).RunControlSvc.func1()
      /home/shanemcd/receptor/pkg/controlsvc/controlsvc.go:345 +0xc4

Previous read at 0x00c00134b4b0 by goroutine 1643:
  runtime.chansend()
      /usr/lib/golang/src/runtime/chan.go:159 +0x0
  github.com/ansible/receptor/pkg/netceptor.(*Netceptor).handleMessageData()
      /home/shanemcd/receptor/pkg/netceptor/netceptor.go:1650 +0x83e
  github.com/ansible/receptor/pkg/netceptor.(*Netceptor).runProtocol()
      /home/shanemcd/receptor/pkg/netceptor/netceptor.go:1879 +0xedb
  github.com/ansible/receptor/pkg/netceptor.(*Netceptor).AddBackend.func1.2()
      /home/shanemcd/receptor/pkg/netceptor/netceptor.go:536 +0xfb