Performance: Add default DKIM configuration to MailerBuilder and performance improvement

[bbottema]

There's something to be said for avoiding a performance hit by moving the DKIM construction to the MailerBuilder so this is done only once rather than for each separate email.

Original suggestion by @koen-serry

[bbottema]

I've refactored the library a little bit so that there now is a special config group on the Mailer level. Email validation criteria moved there and I've added default S/MIME signing in there as well. Next step would be to finally add default DKIM signing in there as well. Will work on this soon.