Two pings required go get NSStreamEventEndEncountered

[wtmoose]

We're seeing that it frequently requires two pings to get NSStreamEventEndEncountered after the app has been suspended for a long time and is awoken by VOIP push. Since pings are 5 seconds apart, there is an additional 5 seconds delay reconnecting to the server.

Our current naive workaround is to make _pingTimerFired public, allowing us to make the second ping much sooner. This consistently reduces the delay to around 0.25 seconds.