LOGBACK-909 Fix race condition in SMTPAppenderBase

This is a fix for http://jira.qos.ch/browse/LOGBACK-909 (SMTPAppender synchronization problem in Asynchronous mode).

SMTPAppenderBase re-uses a single MimeMessage instance for all mails, which caused various problems in asynchronous mode, as MimeMessage is not thread safe. This patch modifies SMTPAppenderBase to create a new MimeMessage instance for each outgoing mail.