Fix racing in Open

[Zerpet]

Summary

• Lock allocator assignment in openComplete
• Add a critical section in openTune

Description

In #170, it was reported a data race in Connection.Open. The situation
described is an edge case, where the connection reader (responsible for reading
data sent from the server/wire) initiates the shutdown sequence in
Connection.shutdown before Connection.Open completes. This creates a data
race for Connection.allocator and Connection.Config.ChannelMax fields.

The shutdown function is already protected by the Connection struct mutex. The
access from Open stack calls is not protected. This PR adds two new, as small
as possible, critical sections protected by the struct mutex during Open.

It is quite challenging to add a reliable test for this edge case, even with a
server fake, since we have to inject a failure just before the frame reader
reads, but right after the Tune frame is sent. Original reporter validated the
fix.

Closes #170

[lukebakken]

Why do we bother re-initializing these two fields when the connection is shutdown (and thus should never be used again)?

https://github.com/rabbitmq/amqp091-go/blob/issue-170/connection.go#L554-L555

[Zerpet]

I wish the past authors of a79bd14 would have left better commit messages to explain why this was necessary 🥲 Earlier, the critical section was smaller and those fields were not modified, looking back at:

amqp091-go/connection.go

Lines 370 to 408 in 425d3a7

	func (c *Connection) shutdown(err *Error) {
	atomic.StoreInt32(&c.closed, 1)
	c.destructor.Do(func() {
	c.m.Lock()
	closes := make([]chan *Error, len(c.closes))
	copy(closes, c.closes)
	c.m.Unlock()
	if err != nil {
	for _, c := range closes {
	c <- err
	}
	}
	for _, ch := range c.channels {
	c.closeChannel(ch, err)
	}
	if err != nil {
	c.errors <- err
	}
	// Shutdown handler goroutine can still receive the result.
	close(c.errors)
	c.conn.Close()
	for _, c := range closes {
	close(c)
	}
	for _, c := range c.blocks {
	close(c)
	}
	c.m.Lock()
	c.noNotify = true
	c.m.Unlock()
	})
	}