Use BeginFlush in .Net 4.5 to prevent events from getting lost under extreme load

[mtscout6]

The default Flush method called in the ServerEventWriter is not thread safe and thus can have the nasty side effect of losing events when under a heavy load.

There is no good solution for this with .Net 4.0, but 4.5 introduces a BeginFlush method. Change the event writer to use this new method.