Deadlock in example_client when publish

[CMonoceros]

Hi, I meet a deadlock when using the latest example.

Although increasing the size of notifyConfirm (https://github.com/streadway/amqp/pull/406), but the producer is blocked when resend more than once between one resend period. I test with one nanosecond resend delay and every time repeat it. Although in a production environment, it hard to realize it. But I think this example has a design defect.

Goroutine stack:

goroutine profile: total 9
2 @ 0x10323c0 0x10426b0 0x104269b 0x1042417 0x1078ecc 0x12494d6 0x1249011 0x12d7ca9 0x12d7927 0x12d9d2a 0x105f671
#	0x1042416	sync.runtime_SemacquireMutex+0x46					/usr/local/Cellar/go/1.13.4/libexec/src/runtime/sema.go:71
#	0x1078ecb	sync.(*Mutex).lockSlow+0xfb						/usr/local/Cellar/go/1.13.4/libexec/src/sync/mutex.go:138
#	0x12494d5	sync.(*Mutex).Lock+0x535						/usr/local/Cellar/go/1.13.4/libexec/src/sync/mutex.go:81
#	0x1249010	github.com/streadway/amqp.(*Channel).Publish+0x70			/Users/zhangjunming/Code/GOPATH/pkg/mod/github.com/streadway/[email protected]/channel.go:1331

1 @ 0x10323c0 0x1006b7b 0x1006b51 0x1006935 0x1249c77 0x1249ee0 0x12476b3 0x124793f 0x124bbb3 0x124b687 0x124be44 0x105f671
#	0x1249c76	github.com/streadway/amqp.(*confirms).confirm+0x96	/Users/zhangjunming/Code/GOPATH/pkg/mod/github.com/streadway/[email protected]/confirms.go:45
#	0x1249edf	github.com/streadway/amqp.(*confirms).Multiple+0x7f	/Users/zhangjunming/Code/GOPATH/pkg/mod/github.com/streadway/[email protected]/confirms.go:79
#	0x12476b2	github.com/streadway/amqp.(*Channel).dispatch+0xa62	/Users/zhangjunming/Code/GOPATH/pkg/mod/github.com/streadway/[email protected]/channel.go:312
#	0x124793e	github.com/streadway/amqp.(*Channel).recvMethod+0x23e	/Users/zhangjunming/Code/GOPATH/pkg/mod/github.com/streadway/[email protected]/channel.go:351
#	0x124bbb2	github.com/streadway/amqp.(*Connection).dispatchN+0xd2	/Users/zhangjunming/Code/GOPATH/pkg/mod/github.com/streadway/[email protected]/connection.go:477
#	0x124b686	github.com/streadway/amqp.(*Connection).demux+0x86	/Users/zhangjunming/Code/GOPATH/pkg/mod/github.com/streadway/[email protected]/connection.go:436
#	0x124be43	github.com/streadway/amqp.(*Connection).reader+0xf3	/Users/zhangjunming/Code/GOPATH/pkg/mod/github.com/streadway/[email protected]/connection.go:528

1 @ 0x10323c0 0x10426b0 0x104269b 0x1042417 0x1078ecc 0x1249bca 0x1249bd0 0x1249335 0x12d7ca9 0x12d7927 0x12d9d2a 0x105f671
#	0x1042416	sync.runtime_SemacquireMutex+0x46					/usr/local/Cellar/go/1.13.4/libexec/src/runtime/sema.go:71
#	0x1078ecb	sync.(*Mutex).lockSlow+0xfb						/usr/local/Cellar/go/1.13.4/libexec/src/sync/mutex.go:138
#	0x1249bc9	sync.(*Mutex).Lock+0xa9							/usr/local/Cellar/go/1.13.4/libexec/src/sync/mutex.go:81
#	0x1249bcf	github.com/streadway/amqp.(*confirms).Publish+0xaf			/Users/zhangjunming/Code/GOPATH/pkg/mod/github.com/streadway/[email protected]/confirms.go:32
#	0x1249334	github.com/streadway/amqp.(*Channel).Publish+0x394			/Users/zhangjunming/Code/GOPATH/pkg/mod/github.com/streadway/[email protected]/channel.go:1360