move from MailDateFormat (not thread safe) to DateTimeFormatter

With high load and usage of MimeMessage plenty of locks due to MailDateFormat. Maybe it is time move to something thread safe like DateTimeFormatter?

Start by locating the MailDateFormat usage in the MimeMessage implementation and inspect how date formatting is synchronized. Verify the replacement preserves existing message formatting and removes the reported lock contention under high load; the issue does not name tests or files to run.

Written by the indexing model from the issue text.