Fix connection lifetime semantics

[davidfowl]

• Make sure things each of the transports disconnect when the browser is closed (http/javascript)
• Make sure events happen in the right order (disconnect during a send, send after disconnect)
• Handle server shutdown
• Make sure all memory is released when:
  • The user code loop ends
  • The transport ends (client disconnects)
• Per connection Timeouts - Long polling only
• Server initiated disconnects.

[davidfowl]

Make sure things each of the transports disconnect when the browser is closed (http/javascript)

This doesn't work great because we can't detect client FIN when running on Kestrel. WebListener works though. If we don't write to the output the FIN from the server is never sent and the RequestAborted token never fires.