[michalovits]

Make sure we don't send channel.open twice. Here is an example race:

• Receive channel.close, transition to CLOSED
• Complete some other write, schedule channel reopen; we're still CLOSED
• Complete the channel.close-ok write, schedule channel reopen
• We send out two opens

More generally:

• Do not open the channel if we're already in an open, opening, or closing state
• Do not reopen the channel if we've moved on from the lifetime we were trying to reopen
• Only schedule a reopen after we've written the close-ok -- see the race above, we were doing it too soon.