Mailbox empty statistics

[samanbarghi]

This line can be reached even when the mailbox is suspended and not empty : https://github.com/AsynkronIT/protoactor-go/blob/dev/mailbox/mailbox.go#L94

[rogeralsing]

Good catch!

[alex023]

Before execution, adding judge sentences?just like：

if !m.suspended{  //<---add this line
 for _, ms := range m.mailboxStats {
		ms.MailboxEmpty()
	}
}