Avoid race between multi-message ack and publish

PR ruby-amqp#617 introduced an optimization for multi-message acks but failed
to protect @unconfirmed_set. In edge cases #basic_publish would
attempt to modify it during iteration with Enumerable#min:

    <ruby>/lib/ruby/3.1.0/set.rb:522:in `add': can't add a new key
    into hash during iteration (RuntimeError)
    from <bunny>/lib/bunny/channel.rb:555:in `block in basic_publish'
    from <bunny>/lib/bunny/channel.rb:554:in `synchronize'
    from <bunny>/lib/bunny/channel.rb:554:in `basic_publish'
    from <bunny>/lib/bunny/exchange.rb:141:in `publish'

lib/bunny/channel.rb

@@ -1788,12 +1788,12 @@ def handle_basic_return(basic_return, properties, content)
 
     # @private
     def handle_ack_or_nack(delivery_tag_before_offset, multiple, nack)
-      delivery_tag          = delivery_tag_before_offset + @delivery_tag_offset
-      confirmed_range_start = multiple ? @delivery_tag_offset + @unconfirmed_set.min : delivery_tag
-      confirmed_range_end   = delivery_tag
-      confirmed_range       = (confirmed_range_start..confirmed_range_end)
-
       @unconfirmed_set_mutex.synchronize do
+        delivery_tag          = delivery_tag_before_offset + @delivery_tag_offset
+        confirmed_range_start = multiple ? @delivery_tag_offset + @unconfirmed_set.min : delivery_tag
+        confirmed_range_end   = delivery_tag
+        confirmed_range       = (confirmed_range_start..confirmed_range_end)
+
         if nack
           @nacked_set.merge(@unconfirmed_set & confirmed_range)
         end