fix(server): avoid race conditions in Run/Stop

- don't close ready channel until PeerServer is listening.
  avoids possible panic in Stop() if PeerServer is nil.

- avoid data race in Run() (err variable was shared between 2 goroutines)

- avoid data race in PeerServer Start/Stop (PeerServer.closeChan)
Merge pull request #725 from dougm/server-lifecycle-fixes

fix(server): avoid race conditions in Run/Stop