go: allow concurrent Write calls

[djs55]

The net.Conn interface says:

// Multiple goroutines may invoke methods on a Conn simultaneously.

Previously concurrent calls to Write() would block for window space but only one would be woken up by a Signal() when a window update is received, allowing the other call to block forever.

The handling of write timeouts was also confusing because

• time.Sleep couldn't be interrupted
• the condition variable Wait() was on a temporary goroutine