Mailtimestamp

Here are some tests for the MailTimeStamp function and also some code improvements. 

While not my primary goal, benchmarks show the new version is more than 60% faster... it's almost too bad OTRS does not use this function more often!

````
                    Rate OldMailTimeStamp NewMailTimeStamp
OldMailTimeStamp 18235/s               --             -64%
NewMailTimeStamp 50601/s             177%               --
````