Add 'wait time before validate' after creating dns record

[BasConijn]

At out company we had some issues that the certificates were no longer renewed. After some trail and error I reduced the problem to creating the TXT record and LetsEncrypt using this value. Without any delay the you will receive an AcmeException from the Validate function. Because of the exception the retry logic 90 seconds does not work either.

[BasConijn]

I used a TimeSpan because the TimeBeforeExpiryToRenew was also a TimeSpan.