move from MailDateFormat (not thread safe) to DateTimeFormatter

[valenpo]

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

[jmehrens]

MailDateFormatter has lots of exceptions for parsing that make the move to DateTimeFormatter not viable.

I think the best fix here is to remove the lock and static field. Then have each call create a method local instance.

Thoughts @anthonyvdotbe?

[anthonyvdotbe]

@jmehrens yes, I agree that your proposal is the best option here